Question

一个多边形工有20条对角线,求它的边数?

Answer 1

n边形对角线有n(n-3)/2条

n(n-3)/2=20

n(n-3)=40

n^2-3n-40=0

(n+5)(n-8)=40

n≠-5

∴n=8

它的边数8