# maths

posted by
**ashely**
.

r1 and r2 are unit vectors in the x-y plane making angles a and b with the positive x-axis.by considering r1.r2 derive

cos(a-b)=cos(a)cos(b)+sin(a)sin(b)

posted by
**ashely**
.

r1 and r2 are unit vectors in the x-y plane making angles a and b with the positive x-axis.by considering r1.r2 derive

cos(a-b)=cos(a)cos(b)+sin(a)sin(b)