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# C3 Trig - 'R formula' Watch

1. Okay, let's say I have two equations:

,

I know to work out the value of R, you square the equations, then add them, as follows:

,

But what I don't understand is, if I go down a different route (which should still work) I get a different answer:

- nothing wrong with that right? Now if squared it just like that, it would be fine. But if I actually find the sum of 5 and 6, then square it, it goes weird:

What am I doing which is 'nonsensical'? From my point of view, I can't see anything that I have done wrong, but I end up with the wrong answer >< .
2. Squaring the RHS side would give not
3. You left out the "2R(sin a + cos a)" when squaring
4. (Original post by ViralRiver)
Okay, let's say I have two equations:

,

I know to work out the value of R, you square the equations, then add them, as follows:

,

But what I don't understand is, if I go down a different route (which should still work) I get a different answer:

- nothing wrong with that right? Now if squared it just like that, it would be fine. But if I actually find the sum of 5 and 6, then square it, it goes weird:

What am I doing which is 'nonsensical'? From my point of view, I can't see anything that I have done wrong, but I end up with the wrong answer >< .
Remember

hope i helped
Good Luck in your C3 exam!
5. Ahh okay. Just to make sure, since it seems I've been fooled by my teachers. if , then does or ? I've always been taught the former (i.e square everything individually) but I've always thought it was the latter >< .
6. (Original post by ViralRiver)
This is correct. NEVER ever do the first one.
7. Thanks. I hate my GCSE teacher xD .
8. I did it by solving them as simultaneous equations:

therefore

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