find cos 165 no calc Watch
thats just stating a formula.....
noooooooooo lol... but they might ask you a question where someone says "Rachel says:
cos(this) + cos(that) is the same as cos(this + that), therefore cos(A) + cos(B) must always equal cos(A + B).
Prove rachel is wrong in her assumption"
All you do is choose two numbers where that doesn't work.
Cos (45 + 120) = Cos 45 Cos 120 - Sin 45 Sin 120.
(If you think about it, Cos 90 = 0.
So if Cos 45 + Cos 45 = Cos 90
then you're saying that which isn't true. )
Whenever you're finding Cos (one number + another number)
or Sin (one number + another number)
then you have to use the formulae for Cos(A+B) etc.
A= 180 degrees, B = 60 degrees, so that A + B = 240 degrees, then
cos[A + B] = cosA + cosB.
In general it is not true that cos[A + B] = cosA + cosB, but there are occasions when it is true. Similarly for sin[A + B] etc.