You're minimizing the function 
where 
.
Take the derivative, set to zero and find the critical points:
This has two roots,
, but we ignore the negative root.
Check the signs of the derivative to the left and right of
and you'll see that 
for 
, and 
for 
, which means a minimum occurs at .
Take the derivative, set to zero and find the critical points:
This has two roots,
Check the signs of the derivative to the left and right of