perhaps this should go in the maths section, but since i read it out of a physics book i'll put it here.

i read in a physics book one day you see and came across an equation for addition of vectors basically saying, when vectors A and B are added the resultant vector is equal to:

R<sup>2</sup>=A<sup>2</sup>+B<sup>2</sup>+2(A)(B)(COS a)

the first part i can see as being pythagorus and only applying to addition at right angles, so by logical deduction the second part is for addition at non-right angles.

can someone explain why we add 2(A)(B)(COS a) to obtain the answer?