Hello, viciouspoultry!

I don't remember the derivation, so I came up with one.

It's rather clumsy, but it's valid.

All I only need to see one of the formulas derived step by step.

Consider: . .[1]

Consider: . .[2]

Multiply [1] and [2]: .

. .

. .

. .

We have: .

. . . . . . . .

Multiply by 2: .