C2 Trig help

Question 7 a and b if possible.
https://c4a3f001dcd45afe69d0ceec83003f9fbc283b2c.googledrive.com/host/0B1ZiqBksUHNYSGtmV3dzVVVvNTg/June%202014%20QP%20-%20C2%20Edexcel.pdf

I can never understand how to do these type of qestions. I would be very grateful if someone could give me a bit of guidance on it. Much appreciated.
Thanks!

Reply 1

PossiblyNotGod

20

For the first one: sin(theta+60)=4/9; sin^-1(4/9) = theta+60

you want to make the range (0 +60) </= (theta+60) < (360 + 60)

then take 60deg away from your results.

For the second: if you times by cosx you get 2sinx - 3 sinxcosx = 0;
Put in another form, sinx(2-3cosx)=0; Therefore sinx = 0 or cosx=2/3

(edited 7 years ago)

Reply 2

B_9710

18

For part b if you factorise to give

\sin x(\frac{2}{\cos x} - 3 ) = 0

then it should become a lot more clear.

\sin x = 0

or

\frac{2}{\cos x} - 3=0

.

What you should not do in this situation is divide by sinx as you will lose answers. You should try and factorise as I have done.

(edited 7 years ago)

Reply 3

Thomith

10

Looks like i was beaten to it but i did this paper earlier and got full marks on this question so here is how i got part b. Feel free to ask any further questions if you have any :3

Reply 4

A.A.M.I.R

Cosx=3/2 sinx=0 find both answers in radians to get values and then make them negative to get two more values. The answers would the ones in the range.

Reply 5

Atiq629

OP

8

Original post by Thomith

Looks like i was beaten to it but i did this paper earlier and got full marks on this question so here is how i got part b. Feel free to ask any further questions if you have any :3

Could you possibly do it for part a as well please.

Reply 6

Thomith

10

Heres what i got, but i didnt show much of my working out. Basically you Sin-1 both sides (assuming you only have a "1sin" notice how i divided by 9 first). Then change the limits depending on what accompanies the "Theta". Then because the answer is positive, only use parts of CAST where Sin is positive (A and S) and then -60 both sides and only put down answers between the original limits.
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Reply 7

Atiq629

OP

8

Original post by Thomith

Heres what i got, but i didnt show much of my working out. Basically you Sin-1 both sides (assuming you only have a "1sin" notice how i divided by 9 first). Then change the limits depending on what accompanies the "Theta". Then because the answer is positive, only use parts of CAST where Sin is positive (A and S) and then -60 both sides and only put down answers between the original limits.
Posted from TSR Mobile

Could you explain how you got those values of theta+60 and then the two values for theta at the end please.