Question 537986
The total area of a right circular cylinder is 42 pi.
 If the height of the cylinder is 4, find the length of the radius.
:
2({{{pi*r^2}}}) + {{{2*pi*r*4}}} = {{{42*pi}}}
divide by {{{2*pi}}}, results:
r^2 + 4r = 21
A quadratic equation
r^2 + 4r - 21 = 0
Factors to
(r+7)(r-3) = 0
the positive solution is all we want here
r = 3 is the radius