The instantaneous rate of change is simply equivalent to
the first derivative of the equation or function. We are given the equation of
Surface Area (A) with respect to side (x):
A = 6 x^2
Taking the first derivative of the equation:
dA = 12 x dx
dA / dx = 12 x
Now the term dA / dx is the instantaneous rate of change
in the surface area with respect to the side length. To get the rate of change
when the side x = a, simply plug this in into the equation:
rate of change = dA / dx = 12 x
so when x = a:
rate of change = 12 a
