problem: If the sum of the squares of nonnegative real numbers $a,b,$ and $c$ is $13$, and $ab + bc + ca = 6$, then what is the sum of $a,b,$ and $c$?
solution: Since $$(a+b+c)^2 = a^2 + b^2 + c^2 + 2ab + 2bc + 2ca = (13) + 2(6) = 25,$$ it follows that $a+b+c = \pm 5$. Since $a,b,c \ge 0$ we find $a+b+c=\boxed{5}$.