Let a and B be non-zero vectors and B = 1 vector a, and the angle between B is π / 4, then find Lim t → 0 (a + TB - a) / T

Let a and B be non-zero vectors and B = 1 vector a, and the angle between B is π / 4, then find Lim t → 0 (a + TB - a) / T

See the picture. Typing is too messy