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C3 trig identities help - how to stop getting too many solutions?

On some exam questions, when solving equations involving trig identities, I multiple solutions in the given range. Some of these are right, but sometimes I get extra solutions which aren't on the mark scheme; the mark schemes also deduct marks for extra solutions - usually only 1 or 2, but I'd rather not lose marks for stupid, silly mistakes.

Is there any way to stop getting these extra solutions? How do you know which ones are unnecessary/wrong?
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Notnek
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Original post by sleepyspider
On some exam questions, when solving equations involving trig identities, I multiple solutions in the given range. Some of these are right, but sometimes I get extra solutions which aren't on the mark scheme; the mark schemes also deduct marks for extra solutions - usually only 1 or 2, but I'd rather not lose marks for stupid, silly mistakes.

Is there any way to stop getting these extra solutions? How do you know which ones are unnecessary/wrong?

Can you post an example question with your working?
Original post by sleepyspider
On some exam questions, when solving equations involving trig identities, I multiple solutions in the given range. Some of these are right, but sometimes I get extra solutions which aren't on the mark scheme; the mark schemes also deduct marks for extra solutions - usually only 1 or 2, but I'd rather not lose marks for stupid, silly mistakes.

Is there any way to stop getting these extra solutions? How do you know which ones are unnecessary/wrong?


Are you not just putting answers that are outside the limits given?
Original post by kieran12321LFC
Are you not just putting answers that are outside the limits given?


Nope, I'm working within the limits. I can read, you know.
Original post by sleepyspider
Nope, I'm working within the limits. I can read, you know.


Alright son, calm your ****

Maybe try drawing the graph of the thing you're tryna solve to give you an idea of where abouts your answers should be
Original post by notnek
Can you post an example question with your working?


Here's one from the C3 book. I got both 60 and 120, but 120 is an extra solution according to the answers.

a) Solve in the interval 0 < x < 180°
i) cosec(x) = 2cot(x)

I've tried solving it using both sin and tan, but I get 60 and 120 for both.
Original post by kieran12321LFC
Alright son, calm your ****

Maybe try drawing the graph of the thing you're tryna solve to give you an idea of where abouts your answers should be


Sorry :colondollar:

Thought I might have to resort to that. I hate graphs so much but ah well, I'll give it a try.
Original post by sleepyspider
Sorry :colondollar:

Thought I might have to resort to that. I hate graphs so much but ah well, I'll give it a try.


what uni/course are you applying for
Original post by kieran12321LFC
what uni/course are you applying for


Physics at either Surrey or Glasgow :smile:
Original post by sleepyspider
Here's one from the C3 book. I got both 60 and 120, but 120 is an extra solution according to the answers.

a) Solve in the interval 0 < x < 180°
i) cosec(x) = 2cot(x)

I've tried solving it using both sin and tan, but I get 60 and 120 for both.


You are possibly rearranging wrong, I got cos(x) = 1/2 and that gave 1 value in that domain of 60.

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I was gonna apply to do theoretical physics at them two but i've been changing what i want to do every other week
sinx( 2cosx - 1 ) = 0

cosx = 1/2

sinx = 0
Original post by Ben4
You are possibly rearranging wrong, I got cos(x) = 1/2 and that gave 1 value in that domain of 60.

Posted from TSR Mobile


I just tried it with cos(x)=1/2 and that worked for me too, thank you!

I saw cosec(x) and cot(x) and thought, nah cos(x) won't apply here. Stupid me
Original post by kieran12321LFC
I was gonna apply to do theoretical physics at them two but i've been changing what i want to do every other week


Same hahaha but I decided that just plain ol' Physics is the safest bet, because you can specialise or switch later on
Original post by the bear
sinx( 2cosx - 1 ) = 0

cosx = 1/2

sinx = 0


Thank you!

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