Discussion Forum



1)

The edge of a cube is decreasing at the rate of 0.04 cm/sec . If the edge of the cube  is 10 cms, then the rate of decrease of surface area of the cube is 


A) 4.8 $cm^{2}/sec$

B) 4.08 $cm^{2}/sec$

C) .48 $cm^{2}/sec$

D) 4.008 $cm^{2}/sec$

Answer:

Option A

Explanation:

Let edge of  cube be x cm, then surface area of the cube , A=6 x2

 It is given that,  $\frac{dx}{dt}=-0.04 cm/sec$

 Now,   $\frac{dA}{dt}=12 x\frac{dx}{dt}$

  = 12 x (-0.04)

 =-0.48 x

 when ,x=10, then $\frac {dA}{dt}$=-0.48 x 10= -4.8 cm2/sec