find cos 165 no calc

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Reply 20

RightSaidJames

milan5baros

thats just stating a formula.....

No it's not, it's telling you flat out that, in general terms, one does not equal the other, so you can't assume that.

Reply 21

chr15chr15

RightSaidJames

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.

o i was gonna say!

Reply 22

milan5baros

well I didn't understand you. But I've cleared it up now.

Reply 23

grace_

Cos 165 = Cos (45 + 120)
Cos (45 + 120) = Cos 45 Cos 120 - Sin 45 Sin 120.

(If you think about it, Cos 90 = 0.

Cos 45 + Cos 45 = \frac{1}{\sqrt 2} + \frac{1}{\sqrt 2} = \frac{2}{\sqrt 2}

So if Cos 45 + Cos 45 = Cos 90
then you're saying that

\frac{2}{\sqrt 2} = 0

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.

Reply 24

Maths Buster

RightSaidJames

Because

cos(A \pm B) \neq cosA \pm cosB

Just to put a spanner in the works, if
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.