Question

two concentric circles are expanding in such a way that the radius of the inner circle is increasing at the rate of 10cm/sec and that of outer circle at the rate of 7cm/sec. at a certain time the radii of the inner and outer circles are respectively 24cm and 30 am. at what time,is the area between the circles increasing or decreasing? how fast?

Answer 1

A=pi*r^2
differntiating both sides w.r.t. t
dA/dt=2*pi*r*dr/dt
now u can calculate dA/dt knowing value of pi=3.14, r=24 & dr/dt =10
similarly 4 2nd case.

Answer 2

thanks for help but how to know circle increasing or decreasing?

Answer 3

r u sure this is the complete question??

Answer 4

@kritima sign will come accordingly u put the sign of change of dr/dt
u can take +ve for increasing - for decreasing.
if ans. comes +ve it is increasing or else otherwise