# Math Help - Trigonometry identities help

1. ## Trigonometry identities help

Section B

Number 9) II

What i did
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cos2x=1-2sin275

cos 2x = cos (x+x)

cos (x+x) = cosA*CosB-SinA*SinB

cos( 60 + 90)= cos60*cos90 - sin60*cos90

section B part III number 9
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cos 2x = cos2x - sin2x

cos( x + x) = cosA*CosB-SinA*SinB

cos(90 + 45)= cos90*cos45-sin90*sin45

I attempted this one similarly.. wrong answer... help!

2. ## Re: Trigonometry identities help

9. II: Note that $1 - 2 \sin^2 {75^{\circ}} = \cos(150^{\circ})$.

3. ## Re: Trigonometry identities help

I know, sorry i made a mistake in labelling and typing , now fixed,

4. ## Re: Trigonometry identities help

Oh okay, I was a little confused with the 45/60 thing.

Also, $\sin(60^{\circ}) = \frac{\sqrt{3}}{2}$, not $\frac{2}{\sqrt{3}}$. The sine/cosine of an angle can only be from -1 to 1 inclusive. Since $-\frac{2}{\sqrt{3}}$ is outside the set [-1,1], this answer cannot be correct.

5. ## Re: Trigonometry identities help

got it! thanks !