Question

The radius of a circle is uniformly increasing at the rate of $$3\,cm/s.$$  What is the rate of increase in area, when the radius is $$10\,cm\,?$$

A. $$6\pi \,c{m^2}/s$$
B. $$10\pi \,c{m^2}/s$$
C. $$30\pi \,c{m^2}/s$$
D. $$60\pi \,c{m^2}/s$$  
Answer :   $$60\pi \,c{m^2}/s$$
Solution :
$$\eqalign{ & {\text{Given}}\,\frac{{dr}}{{dt}} = 3 \cr & {\text{Let }}A = {\text{Area of circle}} = \pi {r^2} \cr & \therefore \,\frac{{dA}}{{dt}} = 2\pi r.\frac{{dr}}{{dt}} = 6\pi r \cr & {\text{Now, }}{\left. {\frac{{dA}}{{dt}}} \right|_{r = 10}} = 6 \times 10 \times \pi = 60\pi \,c{m^2}/s \cr} $$

Releted MCQ Question on
Calculus >> Application of Derivatives

Releted Question 1

If  $$a + b + c = 0,$$    then the quadratic equation $$3a{x^2}+ 2bx + c = 0$$     has

A. at least one root in $$\left[ {0, 1} \right]$$
B. one root in $$\left[ {2, 3} \right]$$  and the other in $$\left[ { - 2, - 1} \right]$$
C. imaginary roots
D. none of these
Releted Question 2

$$AB$$  is a diameter of a circle and $$C$$ is any point on the circumference of the circle. Then

A. the area of $$\Delta ABC$$  is maximum when it is isosceles
B. the area of $$\Delta ABC$$  is minimum when it is isosceles
C. the perimeter of $$\Delta ABC$$  is minimum when it is isosceles
D. none of these
Releted Question 3

The normal to the curve $$x = a\left( {\cos \theta + \theta \sin \theta } \right),y = a\left( {\sin \theta - \theta \cos \theta } \right)$$        at any point $$'\theta '$$ is such that

A. it makes a constant angle with the $$x - $$axis
B. it passes through the origin
C. it is at a constant distance from the origin
D. none of these
Releted Question 4

If $$y = a\ln x + b{x^2} + x$$     has its extremum values at $$x = - 1$$  and $$x = 2,$$  then

A. $$a = 2,b = - 1$$
B. $$a = 2,b = - \frac{1}{2}$$
C. $$a = - 2,b = \frac{1}{2}$$
D. none of these

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