Figure 8. Because the trees pictured here share a root zone, calculating the amount of fertilizer for the entire area is more accurate. In this case, you would calculate the area of a circle encompassing the drip lines of all the trees as one, using the same formula for the area of a circle.
Black-and-white drawing of three adjacent trees with semicircle around base and dotted lines indicating the edges of shared canopy.

Illustration by Barbara Fair.