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Trig equation help

Can someone check this answer for me:

5sin2x = 3cosx where 0<x<90

i get
10sun(x)cos(x) - 3cos(x) = 0
cos(x) (10sin(x) - 3) = 0

so sinx = 3/10

(edited 3 years ago)

Reply 1

TM3141

Original post by Hollymae764

Can someone check this answer for me:

5sin3x = 3cosx where 0<x<90

i get
10sun(x)cos(x) - 3cos(x) = 0
cos(x) (10sin(x) - 3) = 0

so sinx = 3/10

That's not right unfortunately. You haven't dealt with the sin3x correctly i dont think

Reply 2

TM3141

Original post by TM3141

That's not right unfortunately. You haven't dealt with the sin3x correctly i dont think

I also think there must be a typo in this question

Reply 3

PepeTheFroggi

are you sure its not sin2x instead of sin3x?
And if it was sin2x i would divide by cosx to get rid of the cos. Makes solving it easier

(edited 3 years ago)

Reply 4

nerak99

well

5sin2x=3cosx becomes 10sinx cosx =3cosx using the double angle formula. However do NOT divide through by cos .

Rather 10sin x cos x - 3cos x=0 >>> cos x(10sinx-3)=0 This gives cos x=0 and sin x =3/10 as solutions which can be evaluated to angles.

If you divide through by cos you would lose the zero root and probably lose an accuracy mark at least.

If you want to aim for A or A* you can't afford to make the mistakes like dividing through by cos x. This is a distinguishing feature of the question and is used to help fix grade boundaries.