C3 Trig Question

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If I had the equation $x^2 = 2x$, do you agree that it would be folly of us to cancel an $x$ from both sides and get $x = 2$? Surely you know that what you really need to do is $x^2 - 2x = 0 \Rightarrow x(x-2) = 0$ which gets you two solutions.

The same applies with your trigonometric equation. Do not cancel the $\cos$, you should bring it over to the other side and factorise it out whereby you can say that another set of solutions is given by $\cos \theta = 0$.

Or is what I'm saying rubbish and only applicable to this example?

Nope, that's precisely why.

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Thank you, seems obvious now!

You can't cancel the cos in your 3rd line of working. That takes out a solution

Why are you giving advice when its incorrect?

Why are you acting condescending when you haven't yet stated what the "correct" advice would be?

correct advise is to minus cos then factorise