Find `c>0` such that the area between the curves `y=x^2-c^2` and `y=c^2-x^2` is 72.
(1) The graphs:
Note that the intersections of the graphs are at (-c,0) and (c,0). Also note the symmetry about the x-axis. Thus we need only find the area bounded by the x-axis and the curve `y=c^2-x^2` to be 36.
The limits of integration will be -c,c.
(2) `int_(-c)^c c^2-x^2=c^2x-x^3/3 |_(-c)^c`
This represents the area bounded by the x-axis and the curve `y=c^2-x^2` from -c to c. The area of the region of interest is twice this area.
Therefore, for c=3 we have the area between `y=x^9-9` and `y=9-x^2` is 72 as required.