Use intervals to describe the real numbers satisfying the inequalities

Question

If f(x) = x2 - 2x, find f(a+1) and f(a+2)

Answer 1

f(x) = x^2 - 2x = x(x-2)

f(a+1) = (a+1)(a+1-2) = (a+1)(a-1) = a^2 - 1

f(a+2) = (a+2)(a+2-2) = a(a+2)