<h3>Given</h3>
- length, width, and height of a cuboid are x, x, and 2x, respectively
- the cuboid's surface area is 129.6 cm²
- dx/dt = 0.01 cm/s
<h3>Find</h3>
- dV/dt for the given conditions
<h3>Solution</h3>
The equation for surface area can be written
... A = 2(LW +H(L +W))
Substituting the given values gives an equation that we can solve for x
... 129.6 = 2(x·x +2x(x +x)) = 10x²
... x = √(129.6/10) = 3.6 . . . . . . . cm
The equation for volume can be written
... V = LWH
Substituting the given values gives an expression for volume in terms of x.
... V = x·x·2x = 2x³
Then the rate of change of volume is
... dV/dt = 6x²·dx/dt
... dV/dt = 6·(3.6 cm)²·(0.01 cm/s)
... dV/dt = 0.7776 cm³/s
