(40+38+36+...+4+2)-(39+37+35+...+3+1) =40+38+36+...+4+2 - 39-37-35-...-3-1 =40-39+38-37+36-35+……+4-3+2-1 =1+1+1+……+1+1 总共是20个 1 =20
