# Thread: HELP deriving the cosine rule?

1. ## HELP deriving the cosine rule?

Hello there, one of my text books follows that:

a2 = (c-bcosA)^2 + (bsinA)^2

a^2 = c2 -2cb cosA +b2Cos2A +b2sin2A

i start by expanding out the brackets of the first term and distributing, but getting confused.. where does the -2cb come from!? ... thanks in advance!

2. ## Re: HELP deriving the cosine rule?

Use: (a-b)^2=a^2-2.a.b+b^2 so you get:
(c-b.cos(A))^2=c^2-2.c.b.cos(A)+b^2.cos^2(A)