Tag Archives: ISI entrance

Pre-RMO — training

Problem 1:

If a, b, c, and d satisfy the equations





then what is the numerical value of (a+d)(b+c)?

Problem 2:

Suppose x and y are positive integers with x>y and 3x+2y and 2x+3y when divided by 5, leave remainders 2 and 3, respectively. It follows that when x-y is divided by 5, the remainder is necessarily equal to

(A) 2

(B) 1

(C) 4

(D) none of the foregoing numbers

Problem 3:

The number of different solutions (x,y,z) of  the equation x+y+z=10, where each of x, y, and z is a positive integer is

(A) 36

(B) 121

(C) 10^{3}-10

(D) C_{3}^{10}-C_{2}^{10}, which denote binomial coefficients

Problem 4:

The hands of a clock are observed simultaneously from 12.45 pm onwards. They will be observed to point in the same direction some time between

(A) 1:03 pm and 1:04 pm

(B) 1:04 pm and 1:05pm

(C) 1:05 pm and 1:06 pm

(D) 1:06 pm and 1:07 pm.

More later,

Nalin Pithwa


Solutions of Triangles — a tricky IITJEE problem

Question (IITJEE 1978). Suppose p_{1},p_{2},p_{3} are the altitudes through vertices A, B, C of a triangle ABC with area \Delta.

Prove that: 

\frac {1}{p_{1}} + \frac {1}{p_{2}} - \frac {1}{p_{3}}=\frac {2ab}{(a+b+c) \Delta} \cos^{2} (C/2)


The RHS looks daunting. But, if we bring the factor \Delta in its denominator to the LHS, then the problem unfolds itself. Since, \Delta =(1/2)ap_{1}=(1/2)bp_{2}=(1/2)cp_{3}, the problem is equivalent to the following:

prove: a+b-c=\frac {4ab}{a+b+c}\cos^{2}(C/2)

If we write 2\cos^{2}(C/2) as 1+\cos C, and multiply both the sides of by a+b+c, then the problem is reduced to  proving that

(a+b)^{2}-c^{2}=2ab+2ab \cos C.

which is the same as the cosine formula 🙂 🙂 🙂

More later…

Nalin Pithwa