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# C4 Trig Help watch

1. Express Cos(40)Cos(30) + Sin(40)Sin(30) in the form Sin(X)

I have tried putting into the form Cos(A+B) then using the cos+sin=1 identity to work it out but i get no where near the answer. I know it might seem an easy question but im not sure how to go about it.

Thanks for any help
2. The expression is equal to cos(40-30)=cos(10)=sin(80)
Remember that cos(90-θ)=sinθ.
3. (Original post by ABC1123)
Express Cos(40)Cos(30) + Sin(40)Sin(30) in the form Sin(X)

I have tried putting into the form Cos(A+B) then using the cos+sin=1 identity to work it out but i get no where near the answer. I know it might seem an easy question but im not sure how to go about it.

Thanks for any help
http://www.thestudentroom.co.uk/forumdisplay.php?f=38
4. Sin(a) = cos(90-a)
Cos(a) = sin(90-a)

(Im waiting for that cheeky rep)
5. (Original post by B_9710)
The expression is equal to cos(40-30)=cos(10)=sin(80)
Remember that cos(90-θ)=sinθ.
Thanks
6. (Original post by GeologyMaths)
Sin(a) = cos(90-a)
Cos(a) = sin(90-a)

(Im waiting for that cheeky rep)
Thanks, and you sure do get the rep!

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