Let a and b be real numbers such that a + b = -1. Compute a^2 - b^2 + 2a.
a^2 - b^2 + 2a =
(a + b) (a - b) + 2a = {a + b = -1 }
(-1) (a - b) + 2a =
-a + b + 2a =
a + b
And
a + b = -1
