Thread: Trigonometry identities help

Section B

Number 9) II

How do go about answering this?

Please see attached pdf

What i did
________

cos2x=1-2sin²75

cos 2x = cos (x+x)

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

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

answer i got was wrong

section B part III number 9
____________________

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!

9. II: Note that .

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

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

Also, , not . The sine/cosine of an angle can only be from -1 to 1 inclusive. Since is outside the set [-1,1], this answer cannot be correct.

got it! thanks !

what about 9) III? my answer is -1/square root of 2