The formula for the volume of a sphere is $V=\frac{4}{3} \pi r^{3},$ where $r$ represents the radius of the sphere.
(a) Write a function $D(r)$ that gives the volume gained when the radius of a sphere of $r$ inches is increased by 3 inches.
(b) Graph $y=D(r)$ found in part (a), using $x$ for $r,$ in the window $[0,10]$ by $[0,1500]$
(c) Use your calculator to graphically find the amount of volume gained when a sphere with a 4-inch radius is increased to a 7 -inch radius.
(d) Verify your result in part (c) analytically.