I'm using some Anki flash cards to study statistics and have come across an equation I don't recognize:
$$E(X)^2 = \sum r^2 P(X=r)$$
I found a similar equation in the last line of the proof of the Law of total expectation on Wikipedia.
I'm wondering if someone could point me to a place where this equation is described and possibly some applications for which it is used.
The Card deck is named Tyler GCE Maths S1 - Key facts and equations required for the OCR MEI Mathematics Statistics 1 exam.