Boundary Value Problem $y''+uy=0$ Consider the boundary value problem 
$$y''+uy=0  \qquad     y(0)=y(\pi/2)=0$$
(a) For what values of $u$ does this problem have the trivial solution $y \equiv 0$?
(b) For what values of $u$ does the problem have nontrivial solutions?
I would really appreciate any help on this one.

Here is your error: you say "What i had done was sub into the equation $r$, such that $y'' + uy$ becomes: $r^2 + u = 0 r= \pm (-u)^{1/2}$ Then since the root is a complex number: $r= (u)^{1/2}i$".
The root will be a complex number only if u is positive and you do not know that.
Rather, consider the separate cases.
1) $u= 0$.  In this case, the differential equation is $y''= 0$ which can be solved by integrating twice: $y'= A$, a constant, so $y(x)= Ax+ B$.  Now, $y(0)= B= 0$ and $y(\frac{\pi}{2})= A\cdot\frac{\pi}{2}= 0$ so $A= 0$.  Since $A$ and $B$ are both $0$, $y(x)$ is identically $0$, the "trivial" solution.
2) $u< 0$.  Let $u= -a^2$ where $a$ can be any non-zero number.  Then the equation is $y''- a^2y= 0$ which has characteristic equation $r^2- a^2= 0$.  That has real roots $a$ and $-a$.  The general solution to the equation is $y(x)= Ae^{ax}+ Be^{-ax}$.  $y(0)= A+ B= 0$ and $y(\frac{\pi}{2})= Ae^{a\cdot\frac{\pi}{2}}+ Be^{-a\cdot\frac{\pi}{2}}$.  From $A+ B= 0, B= -A$ so that   so that $A- B= 0$ and $A= B$.  Then $Ae^{a\frac{\pi}{2}}+ Be^{-a\frac{\pi}{2}}= Ae^{a\frac{\pi}{2}}- Ae^{-a\frac{\pi}{2}}= A(e^{a\frac{\pi}{2}}- e^{-a\frac{\pi}{2}})= 0$.  Since $a$ is not $0$, $e^{a\frac{\pi}{2}}- e^{-a\frac{\pi}{2}}$ is not $0$ and we must have $A= 0$ and $B= -A= 0$.   Again $A$ and $B$ are both $0$ so the $y(x)$ is identically $0$, the "trivial" solution.
3) $u> 0$.  Let $u= a^2$ where a can be any non-zero number.  Then the equation is $y''+ ay^2= 0$ which has characteristic equation $r^2+ a^2= 0$.  That has imaginary roots $ai$ and $-ai$.  The general solution to the equation is $y(x)= A \cos(ax)+ B \sin(ax)$.  $y(0)= A= 0$ and $y(\frac{\pi}{2})= B \sin(a\frac{\pi}{2})= 0$.  Now, we must have either $B= 0$ so that we have the "trivial" solution again or $\sin(a\frac{\pi}{2})$ which happens if and only if $a$ is an odd number: $a= 2n+ 1$ for some integer $n$.
