Classic Proof for the Cauchy Schwarz inequality. You can also substitute:
x = \dfrac{ab+cd}{a^2 + c^2} into f(x)
Now try this
Use the fact that f(x)=|ax-b|^2+|cx-d|^2\geq 0 to prove |a\bar{b}+c\bar{d}|\leq \sqrt{(|a|^2+|c|^2)(|b|^2+|d|^2)} where a,b,c,d are complex numbers