In most geometry courses, we learn that there's no such thing as "SSA Congruence". That is, if we have triangles $ABC$ and $DEF$ such that $AB = DE$, $BC = EF$, and $\angle A = \angle D$, then we cannot deduce that $ABC$ and $DEF$ are congruent. However, there are a few special cases in which SS...

I haven't checked the specific issue, but be aware that pins are auto-removed after 14 days, because we found this feature to be abused too much. If you feel so strongly about a particular message, why don't you just post it again and pin the new one?

Find the residue at $z=0$ of the function $f(z)=\frac{\cot z}{z^4}$ I know that $z_0=0$ is a pole of order $k=4$, then $\phi(z)=\cot z$ and $$Res(f;z_0)=\frac{\phi(z_0)^{(k-1)}}{(k-1)!}$$ but I cannot get the right answer that is $-\frac{1}{45}$