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# Simultaneous equations? with trig? not sure watch

1. Eliminate theta from the equations: x=cos2theta , y=sectheta

I have no clue what to do here. Is it meant to be like simultaneous equations because i tired and failed
2. Do you know the formulas relating:
• cos(theta) and sec(theta)
• cos(2theta) and cos(theta)
3. (Original post by rayquaza17)
Do you know the formulas relating:
• cos(theta) and sec(theta)
• cos(2theta) and cos(theta)
Well, i know the with cos2theta will be (most likely in this case) 2cos^2theta-1

I am not sure by what you mean by: cos(theta) and sec(theta). I know they sec is 1/cos
4. (Original post by Nirm)
Well, i know the with cos2theta will be (most likely in this case) 2cos^2theta-1

I am not sure by what you mean by: cos(theta) and sec(theta). I know they sec is 1/cos
Yep.

Sec(theta)=1/cos(theta).
So 1/y=cos(theta)
Now if you know x=2cos^2(theta)-1, can you sub the above equation in?
5. (Original post by Nirm)
Well, i know the with cos2theta will be (most likely in this case) 2cos^2theta-1

I am not sure by what you mean by: cos(theta) and sec(theta). I know they sec is 1/cos
That's good. So you know that

and

.

Can you make any progress from there?
6. (Original post by rayquaza17)
Yep.

Sec(theta)=1/cos(theta).
So 1/y=cos(theta)
Now if you know x=2cos^2(theta)-1, can you sub the above equation in?
(Original post by BuryMathsTutor)
That's good. So you know that

and

.

Can you make any progress from there?
Yeah. So i ended up with (2/y^2)-1. Is this it? I thought there would be more to it haha ( if this is the answer then thank you for your help )
7. (Original post by Nirm)
Yeah. So i ended up with (2/y^2)-1. Is this it? I thought there would be more to it haha
Yes, but what values can x take?
8. (Original post by BuryMathsTutor)
Yes, but what values can x take?
Do i have to make y=0 then see what i get for x?If that is the case then i get x=-1
9. (Original post by Nirm)
Do i have to make y=0 then see what i get for x?If that is the case then i get x=-1
No, just note that and or .
10. (Original post by BuryMathsTutor)
No, just note that and or .
Ahh okay, thank you very much!

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Updated: January 10, 2015
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