Help trig

Alevelhelp.1

Help I don’t understand where I went wrong plz help

for part iv

(edited 11 months ago)

Reply 1

mqb2766

Line 2. What happened to 1/cos(2theta)?

One tip for finding where an error occurs is to sub the answer into the equation on each line and check equality holds. As soon as equality breaks, something went wrong. pi/8 satisfies line 2, but not line 1.

(edited 11 months ago)

Reply 2

Alevelhelp.1

Original post by mqb2766

Line 2. What happened to 1/cos(2theta)?

I took the one to the other side and divided by cos theta… wdym I’m confused 😭

(edited 11 months ago)

Reply 3

mqb2766

On line 1 there are three terms. On line 2 there are two terms. Write it (division by cos) out clearly.

I was editing the previous post as you replied, so maybe sub pi/8 into both line 2 and line 1 and check something is screwy.

Reply 4

davros

Original post by Alevelhelp.1

I took the one to the other side and divided by cos theta… wdym I’m confused 😭

You can't go from 1 + C = S to S/C = 1. You need to pay careful attention to these algebraic rearrangements, or you'll create problems for yourself further down the line and end up solving the wrong problem (as you have done here) :smile:

Reply 5

achieve526kk

1+ cos(2theta)=sin(2theta)---> 1+ 2cos(theta)*cos(theta) -1 =2*sin(2theta)cos(theta)

cos(theta)(cos(theata)-sin(theta)=0

theta=arccos(0)= pi/2 and tan(thrta)=1 .....

Reply 6

davros

Original post by achieve526kk

1+ cos(2theta)=sin(2theta)---> 1+ 2cos(theta)*cos(theta) -1 =2*sin(2theta)cos(theta)

cos(theta)(cos(theata)-sin(theta)=0

theta=arccos(0)= pi/2 and tan(thrta)=1 .....

Using double-angle formulae here is a bit overkill tbh. If you've got an equation like 1 + cos(2Y) = sin(2Y) this is exactly the same as solving 1 + cos X = sin X, where X = 2Y. So you can solve the simpler problem in X and then adjust the domains as appropriate :smile:

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