Question

I'm confused as to how they got from "441 = 225 + 256 – 480 cos A" to "-40 = -480 cos A" ? Problem attached

Answer 1

http://www.wolframalpha.com/input/?i=21%5E2+%3D+15%5E2+%2B+16%5E2+%E2%80%93+2*15*16+cos+A

Answer 2

law of cosines

Answer 3

Yes, I know it's the law of cosines. What I don't understand is how they got from this step to that step. Can you explain please?

Answer 4

441 - 225 - 256 = -40

Answer 5

Why isn't it 441-1?

Answer 6

\[441 = 225 + 256 – 480 cos A\]
\[441=481-480\cos(A)\]
\[441-481=-480\cos(A)\]
\[-40=-480\cos(A)\] etc

Answer 7

-480 is the coefficient of \(\cos(A)\)

Answer 8

Oh, so they have to stick together then?

Answer 9

like
\[4=10+6-2x\]
\[4=16-2x\]
\[-12=-2x\] etc

Answer 10

it is -480 times the cosine of A

Answer 11

Oh! Thanks for explaining it like that. I understand noooow :D