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
How to find $\lim_{h\rightarrow 0}\frac{\sin(ha)}{h}$ without lhopital rule? I know when I use lhopital I easy get $$ \lim_{h\rightarrow 0}...
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