Variance of Bernoulli Distribution/Proof 1

Proof 1
From the definition of variance:
 * $\operatorname{var} X = E \left({\left({X - E \left({X}\right)}\right)^2}\right)$

From the Expectation of Bernoulli Distribution, we have $E \left({X}\right) = p$.

Then by definition of Bernoulli distribution: