may overlap a bit with previous lecture(s)…

Mathematics demystified

January 9, 2019 – 4:24 am

may overlap a bit with previous lecture(s)…

November 13, 2018 – 3:16 pm

**Question I:**

Show that the equation of the tangent to the curve and can be represented in the form:

**Question 2:**

Show that the derivative of the function , when and , when vanishes on an infinite set of points of the interval $latex (0,1), Hint: Use Rolle’s theorem.

**Question 3:**

Prove that for . Use Lagrange’s theorem.

**Question 4:**

Find the largest term in the sequence . Hint: Consider the function in the interval .

**Question 5:**

A point P is given on the circumference of a circle with radius r. Chords QR are drawn parallel to the tangent at P. Determine the maximum possible area of the triangle PQR.

**Question 6:**

Find the polynomial of degree 6, which satisfies and has a local maximum at and local minimum at and 2.

**Question 7:**

For the circle , find the value of r for which the area enclosed by the tangents drawn from the point to the circle and the chord of contact is maximum.

**Question 8:**

Suppose that f has a continuous derivative for all values of x and , with for all x. Prove that .

**Question 9:**

Show that , if .

**Question 10:**

Let . Show that the equations has a unique root in the interval and identify it.

November 13, 2018 – 6:26 am

**Question 1.**

If the point on , where , where the tangent is parallel to has an ordinate , then what is the value of ?

**Question 2:**

Prove that the segment of the tangent to the curve , which is contained between the coordinate axes is bisected at the point of tangency.

**Question 3:**

Find all the tangents to the curve for that are parallel to the line .

**Question 4:**

Prove that the curves , where , and , where is a differentiable function have common tangents at common points.

**Question 5:**

Find the condition that the lines may touch the curve .

**Question 6:**

Find the equation of a straight line which is tangent to one point and normal to the point on the curve , and .

**Question 7:**

Three normals are drawn from the point to the curve . Show that c must be greater than 1/2. One normal is always the x-axis. Find c for which the two other normals are perpendicular to each other.

**Question 8:**

If and are lengths of the perpendiculars from origin on the tangent and normal to the curve respectively, prove that .

**Question 9:**

Show that the curve , and is symmetrical about x-axis and has no real points for . If the tangent at the point t is inclined at an angle to OX, prove that . If the tangent at meets the curve again at Q, prove that the tangents at P and Q are at right angles.

**Question 10:
**

Find the condition that the curves and intersect orthogonality and hence show that the curves and also intersect orthogonally.

More later,

Nalin Pithwa.

October 19, 2018 – 6:33 pm

**“Easy” questions:**

Question 1:

Find the slope of the tangent to the curve represented by the curve and at the point .

Question 2:

Find the co-ordinates of the point P on the curve , the tangent at which is perpendicular to the line .

Question 3:

Find the co-ordinates of the point lying in the first quadrant on the ellipse so that the area of the triangle formed by the tangent at P and the co-ordinate axes is the smallest.

Question 4:

The function , where is

(a) increasing on

(b) decreasing on

(c) increasing on and decreasing on

(d) decreasing on and increasing on .

Fill in the correct multiple choice. Only one of the choices is correct.

Question 5:

Find the length of a longest interval in which the function is increasing.

Question 6:

Let , then is

(a) increasing on

(b) decreasing on

(c) increasing on

(d) decreasing on .

Fill in the correct choice above. Only one choice holds true.

Question 7:

Consider the following statements S and R:

S: Both and are decreasing functions in the interval .

R: If a differentiable function decreases in the interval , then its derivative also decreases in .

Which of the following is true?

(i) Both S and R are wrong.

(ii) Both S and R are correct, but R is not the correct explanation for S.

(iii) S is correct and R is the correct explanation for S.

(iv) S is correct and R is wrong.

Indicate the correct choice. Only one choice is correct.

Question 8:

For which of the following functions on , the Lagrange’s Mean Value theorem is not applicable:

(i) , when ; and , when .

(ii) , when ; and , when .

(iii)

(iv) .

Only one choice is correct. Which one?

Question 9:

How many real roots does the equation have?

Question 10:

What is the difference between the greatest and least values of the function ?

More later,

Nalin Pithwa.

December 18, 2017 – 8:43 pm

**Part I: Multiple Choice Questions:**

*Example 1:*

Locus of the mid-points of the chords of the circle which subtend a right angle at the centre is (a) (b) (c) (d)

*Answer 1: C.*

Solution 1:

Let O be the centre of the circle , and let AB be any chord of this circle, so that . Let be the mid-point of AB. Then, OM is perpendicular to AB. Hence, . Therefore, the locus of is .

*Example 2:*

If the equation of one tangent to the circle with centre at from the origin is , then the equation of the other tangent through the origin is (a) (b) (c) (d) .

*Answer 2: C.*

Solution 2:

Since touches the given circle, its radius equals the length of the perpendicular from the centre to the line . That is,

.

Let be the equation of the other tangent to the circle from the origin. Then,

, which gives two values of m and hence, the slopes of two tangents from the origin, with the product of the slopes being -1. Since the slope of the given tangent is -3, that of the required tangent is 1/3, and hence, its equation is .

*Example 3.*

A variable chord is drawn through the origin to the circle . The locus of the centre of the circle drawn on this chord as diameter is (a) (b) (c) (d) .

*Answer c.*

Solution 3:

Let be the centre of the required circle. Then, being the mid-point of the chord of the given circle, its equation is .

Since it passes through the origin, we have .

Hence, locus of is .

**Quiz problem:**

A line meets the coordinate axes in A and B. A circle is circumscribed about the triangle OAB. If m and n are the distances of the tangent to the circle at the origin from the points A and B respectively, the diameter of the circle is (a) (b) (c) (d) .

*To be continued,*

*Nalin Pithwa.*

September 23, 2017 – 11:08 am

I have a collection of some “random”, yet what I call ‘beautiful” questions in Co-ordinate Geometry. I hope kids preparing for IITJEE Mains or KVPY or ISI Entrance Examination will also like them.

Problem 1:

Given n straight lines and a fixed point O, a straight line is drawn through O meeting lines in the points , , , , and on it a point R is taken such that

Show that the locus of R is a straight line.

Solution 1:

Let equations of the given lines be , , and the point O be the origin .

Then, the equation of the line through O can be written as where is the angle made by the line with the positive direction of x-axis and r is the distance of any point on the line from the origin O.

Let be the distances of the points from O which in turn and , where .

Then, coordinates of R are and of are where .

Since lies on , we can say for

, for

…as given…

Hence, the locus of R is which is a straight line.

Problem 2:

Determine all values of for which the point lies inside the triangle formed by the lines , , .

Solution 2:

Solving equations of the lines two at a time, we get the vertices of the given triangle as: , and .

So, AB is the line , AC is the line and BC is the line

Let be a point inside the triangle ABC. (please do draw it on a sheet of paper, if u want to understand this solution further.) Since A and P lie on the same side of the line , both and must have the same sign.

or which in turn which in turn either or ….call this relation I.

Again, since B and P lie on the same side of the line , and have the same sign.

and , that is, …call this relation II.

Lastly, since C and P lie on the same side of the line , we have and have the same sign.

that is

or ….call this relation III.

Now, relations I, II and III hold simultaneously if or .

Problem 3:

A variable straight line of slope 4 intersects the hyperbola at two points. Find the locus of the point which divides the line segment between these two points in the ratio .

Solution 3:

Let equation of the line be where c is a parameter. It intersects the hyperbola at two points, for which , that is, .

Let and be the roots of the equation. Then, and . If A and B are the points of intersection of the line and the hyperbola, then the coordinates of A are and that of B are .

Let be the point which divides AB in the ratio , then and , that is, …call this equation I.

and ….call this equation II.

Adding I and II, we get , that is,

….call this equation III.

Subtracting II from I, we get

so that the locus of is

More later,

Nalin Pithwa.

September 22, 2017 – 3:06 am

Problem 1:

The line joining and is produced to the point so that , then find the value of .

Solution 1:

As M divides AB externally in the ratio , we have and which in turn

Problem 2:

If the circumcentre of a triangle lies at the origin and the centroid in the middle point of the line joining the points and , then where does the orthocentre lie?

Solution 2:

From plane geometry, we know that the circumcentre, centroid and orthocentre of a triangle lie on a line. So, the orthocentre of the triangle lies on the line joining the circumcentre and the centroid , that is, , or . That is, the orthocentre lies on this line.

Problem 3:

If a, b, c are unequal and different from 1 such that the points , and are collinear, then which of the following option is true?

a:

b:

c:

d:

Solution 3:

Suppose the given points lie on the line then a, b, c are the roots of the equation :

, or

and , that is,

Eliminating l, m, n, we get

, that is, option (d) is the answer.

Problem 4:

If and are in A.P., with common difference a and b respectively, then on which line does the centre of mean position of the points with lie?

Solution 4:

Note: Centre of Mean Position is .

Let the coordinates of the centre of mean position of the points , be then

and

,

and

, and

, that is, the CM lies on this line.

Problem 5:

The line L has intercepts a and b on the coordinate axes. The coordinate axes are rotated through a fixed angle, keeping the origin fixed. If p and q are the intercepts of the line L on the new axes, then what is the value of ?

Solution 5:

Equation of the line L in the two coordinate systems is , and where are the new coordinate of a point when the axes are rotated through a fixed angle, keeping the origin fixed. As the length of the perpendicular from the origin has not changed.

or . So, the value is zero.

Problem 6:

Let O be the origin, and and are points such that and , then which of the following options is true:

a: P lies either inside the triangle OAB or in the third quadrant

b: P cannot lie inside the triangle OAB

c: P lies inside the triangle OAB

d: P lies in the first quadrant only.

Solution 6:

Since , P either lies in the first quadrant or in the third quadrant. The inequality represents all points below the line . So that and imply that either P lies inside the triangle OAB or in the third quadrant.

Problem 7:

An equation of a line through the point whose distance from the point has the greatest value is :

option i:

option ii:

option iii:

option iv:

Solution 7:

Let the equation of the line through be . If p denotes the length of the perpendicular from on this line, then

, say

then is greatest if and only if s is greatest.

Now,

so that . Also, , if , and

, if

and , if . So s is greatest for . And, thus, the equation of the required line is .

Problem 8:

The points , , Slatex C(4,0)$ and are the vertices of a :

option a: parallelogram

option b: rectangle

option c: rhombus

option d: square.

*Note: more than one option may be right. Please mark all that are right.*

Solution 8:

Mid-point of AC =

Mid-point of BD =

the diagonals AC and BD bisect each other.

ABCD is a parallelogram.

Next, and and since the diagonals are also equal, it is a rectangle.

As and , the adjacent sides are not equal and hence, it is neither a rhombus nor a square.

Problem 9:

Equations and will represent the same line if

option i:

option ii:

option iii:

option iv:

Solution 9:

The two lines will be identical if there exists some real number k, such that

, and , and .

or

or , and

or

That is, or , or .

Next, . Hence, , or .

Problem 10:

The circumcentre of a triangle with vertices , and lies at the origin, where and . Show that it’s orthocentre lies on the line

Solution 10:

As the circumcentre of the triangle is at the origin O, we have , where r is the radius of the circumcircle.

Hence,

Therefore, the coordinates of A are . Similarly, the coordinates of B are and those of C are . Thus, the coordinates of the centroid G of are

.

Now, if is the orthocentre of , then from geometry, the circumcentre, centroid, and the orthocentre of a triangle lie on a line, and the slope of OG equals the slope of OP.

Hence,

because .

Hence, the orthocentre lies on the line

.

Hope this gives an assorted flavour. More stuff later,

Nalin Pithwa.

July 13, 2017 – 10:45 am

Problem 1: Find the value of when

Problem 2: Reduce the following fraction to its lowest terms:

Problem 3: Simplify:

Problem 4: If , prove that

Problem 5: If a, b, c are in HP, show that .

*May u discover the joy of Math! π π π*

Nalin Pithwa.

May 23, 2017 – 9:04 pm

In the following problems, each year is assumed to be consisting of 365 days (no leap year):

Problem 1:

What is the least number of people in a room such that it is more likely than not that at least two people will share the same birthday?

Solution 1:

The probability of the second person having a different birthday from the first person is . The probability of the first three persons having different birthdays is . In this way, the probability of all n persons in a room having different birthdays is . For the value of n, when P(n) falls just below 1/2 is the least number of people in a room when the probability of at least two people having the same birthday becomes greater than one half (that is, more likely than not). Now, one can make the following table:

Thus, the answer is 23. One may say that during a football match with one referee, it is more likely than not that at least two people on the field have the same birthday! π π π

Problem 2:

You are in a conference. What is the least number of people in the conference (besides you) such that it is more likely than not that there is at least another person having the same birthday as yours?

Solution 2:

The probability of the first person having a different birthday from yours is . Similarly, the probability of the first two persons not having the same birthday as yours is . Thus, the probability of n persons not Β having the same birthday as yours is . When this value falls below 0.5, then it becomes more likely than not that at least another person has the same birthday as yours. So, the least value of n is obtained from . Taking log of both sides, we solve to get . So, the least number of people required is 253.

Problem 3:

A theatre owner announces that the first person in the queue having the same birthday as the one who has already purchased a ticket will be given a free entry. Where (which position in the queue) should one stand to maximize the chance of earning a free entry?

Solution 3:

For the nth person to earn a free entry, first persons must have different birthdays and the nth person must have the same birthday as that of one of these previous persons. The probability of such an event can we written as

For a maximum, we need . Alternatively, . Using this expression for P(n), we get . Or, . For positive n, this inequality is satisfied first for some n between 19 and 20. So, the best place in the queue to get a free entry is the 20th position.

More later,

*Nalin Pithwa.*