Verified answer

u.v = |u| |v| cos W, where W is the angle between u and v.

Cosine law: c^2 = a^2 + b^2 - 2ab cos(C)

cos(C) = (a^2 + b^2 - c^2) / (2ab)

So, the angle between u & v should be C and w will be c in cosine law.

cos(W) = (|u|^2 + |v|^2 - |w|^2) / (2|u| |v|)

cos(W) = (16 + 25 - 36) / 2(4)(5)

cos(W) = (41 - 36) / 40

cos(W) = 5 / 40 = 1/8

u.v = |u| |v| cos W

u.v = 4 * 5 * 1/8 = 20/8

Reduce the fraction

u.v = 5/2 = 2.5