discrete mathematics - Proof by induction: $sum^{2n-1}_{i=1} (2i-1)=(2n-1)^2$

$\sum^{2n-1}_{i=1} (2i-1)=(2n-1)^2$

I get stuck after proving the base case is true. Usually with induction I assume the left and right sides are equal at some k, but I'm not sure how to approach this problem since the left side is a sum.