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trig question c44

So if i had the equation 4cos x + 3 sin x = 5,

I know if i want to solve this, i would have to put it in the form R cos ( x -+ A)
how do i know whether its plus/minus.
Reply 1
Avatar for imran_
imran_
2
Original post by SunDun111
So if i had the equation 4cos x + 3 sin x = 5,

I know if i want to solve this, i would have to put it in the form R cos ( x -+ A)
how do i know whether its plus/minus.


it would be minus
look at the formula sheet and compare it
Reply 2
Avatar for SunDun111
SunDun111
OP
11
Original post by imran_
it would be minus
look at the formula sheet and compare it


but how do you knwo its minus?
Reply 3
Avatar for Zacken
Zacken
22
Original post by SunDun111
So if i had the equation 4cos x + 3 sin x = 5,

I know if i want to solve this, i would have to put it in the form R cos ( x -+ A)
how do i know whether its plus/minus.


Rcos(x-a) = R cos x cos A + R sin x sin A

Rcos(x+a) = R cos x cos a - R sin x sin A

So it is commonplace to choose the +- depending on which one matches the coefficients of your equation. You have +3 so it makes sense to choose +R sin x sin A which means choosing Rcos(x-a).

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