数论杂项

等比数列：\(S_n=a_0\frac{q^n-1}{q-1}\)
n以内质数个数大概是 \(\frac{n}{\ln\ n}\)
三角形叉积 \(S=\frac{|x_1y_2-x_2y_1|}{2}\)
by lyk