2*4+4*6+...+18*20-1*3-3*5-...-17*19简便计算

---

a=2n(2n+2)=4nxn+4n
b=(2n-1)(2n+1)=4nxn-1
c=a-b=4n+1
原野兆式颂消租=4(1+2+3+...+9)+1x9
=4x(1+9)x9/桥宴2+9
=4x45+9
=189

---