## Applications of Derivatives: IITJEE Maths tutorial problem set: III

Slightly difficult questions, I hope, but will certainly re-inforce core concepts:

1. Prove that the segment of the tangent to the curve $y=c/x$ which is contained between the co-ordinate axes, is bisected at the point of tangency.
2. Find all tangents to the curve $y=\cos{(x+y)}$ for $-\pi \leq x \leq \pi$ that are parallel to the line $x+2y=0$.
3. Prove that the curves $y=f(x)$, where $f(x)>0$ and $y=f(x)\sin(x)$, where $f(x)$ is a differentiable function, have common tangents at common points.
4. Find the condition that the lines $x\cos{\alpha} + y \sin{\alpha}=p$ may touch the curve $(\frac{x}{a})^{m} + (\frac{y}{b})^{m}=1$.
5. If $p_{1}$ and $p_{2}$ are lengths of the perpendiculars from origin on the tangent and normal to the curve $x^{2/3} + y^{2/3}=a^{2/3}$ respectively, prove that $4p_{1}^{2}+p_{2}^{2}=a^{2}$.
6. Show that the curve $x=1-3t^{2}$, $y=t-3t^{3}$ is symmetrical about x-axis and has no real points for $x>1$. If the tangent at the point t is inclined at an angle $\psi$ to OX, prove that $3t = \tan{\psi} + \sec{\psi}$. If the tangent at $P(-2,2)$ meets the curve again at Q, prove that the tangents at P and Q are at right angles.
7. A tangent at a point $P_{1}$ other than $(0,0)$ on the curve $y=x^{3}$ meets the curve again at $P_{2}$. The tangent at $P_{2}$ meets the curve at $P_{3}$ and so on. Show that the abscissae of $P_{1}, P_{2}, \ldots, P_{n}$ form a GP. Also, find the ratio of area $\frac{\Delta P_{1}P_{2}P_{3}}{area \hspace{0.1in} P_{2}P_{3}P_{4}}$.
8. Show that the square roots of two successive natural numbers greater than $N^{2}$ differ by less than $\frac{1}{2N}$.
9. Show that the derivative of the function $f(x) = x \sin {(\frac{\pi}{x})}$, when $x>0$, and $f(x)=0$ when $x=0$ vanishes on an infinite set of points of the interval $(0,1)$.
10. Prove that $\frac{x}{(1+x)} < \log {(1+x)} < x$ for $x>0$.

More later, cheers,

Nalin Pithwa.

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