Help urgent maths

Help I don’t get where I went wrong… I know I used sin instead of cos but that doesn’t matter right.. but I still got wrong ans pls help me

its part c

(edited 11 months ago)

Reply 1

mqb2766

It doesnt matter whether its sin or cos, as long as alpha is calculated correctly. You should have both cos(alpha) and sin(alpha) being negative, which would be in quadrant 3 so youd have to be careful when doing atan.

The easiest/safest identity for doing
cos(x) - sin(x)
or similar is
cos(x+alpha) = cos(x)cos(alpha)-sin(x)sin(alpha)
as you know the trig terms cos(alpha) and sin(alpha) must both be +ive so alpha lies in 0...pi/2 and can be safely done using atan.

(edited 11 months ago)

Reply 2

Marvel_fan

Original post by Alevelhelp.1

Help I don’t get where I went wrong… I know I used sin instead of cos but that doesn’t matter right.. but I still got wrong ans pls help me

its part c

Hey, this one took me a while to figure out too!

I believe it should be (pi/3 - x) in the brackets instead of (x - pi/3), since in the first line you are saying sin(α)cos(x) - sin(x)cos(α) which is equivalent to Rsin(α - x)