Quote: r=sqrt(20-r)
r = sqrt(20-r)
r
2 = 20 - r
r
2 + r - 20 = 0
(r + 5)(r - 4) = 0
r = -5, r = 4
but we're not done yet, since we squared both sides we have to check to see that all the solutions are valid for the original equation
4 = sqrt(16) = sqrt(20-4) so r=4 is indeed valid
on the other hand since a square root is never negative -5 is not a valid solution.
Thus the only solution to the original equation is r=4
+6251 
