Thread: Vector dot products problem

If 'a' and 'b' are unit vectors and |a+b| = sqrt(3)
what is (2a-5b) · (b + 3a)?

Hello,

a·a=b·b=1.
(a+b)·(a+b)=3 implies a·a+2a·b+b·b=3.
Once you know a·b, substitute in (2a-5b)·(b + 3a)=2a·b+6a·a-5b·b-15a·b.

Bye.