I need to prove that: $$1+\cos(a+b)-\cos(a-b)=\cos^2(a)+\cos^2(b)$$
I tried to start from both ways but it always took me back to the same one again I tried also to prove that their - equal zero but I didn't work also
I have injection $f \colon A \rightarrow B$ and I want to get bijection. Can I just resting codomain to $f(A)$? I know that every function i...
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