Given a + B = 6, what is 1 / 2A ^ 2 + AB + 1 / 2B ^ 2

Given a + B = 6, what is 1 / 2A ^ 2 + AB + 1 / 2B ^ 2

1/2a^2+ab+1/2b^2
=1/2(a^2+2ab+b^2)
=1/2(a+b)^2
=1/2*6^2=18