(1/1*4)+(1/2*5)+(1/3*6).+(1/17*20)+(1/18*21)=?

(1/1*4)+(1/2*5)+(1/3*6).+(1/17*20)+(1/18*21)=?

균열구화법을 이용하여 하다.
(1/1*4)+ (1/2*5)+(1/3*6).+(1/17*20)+(1/18*21)
=(1/3)×(1-1/4+1/2-1/5+1/3-1/6.+1/17-1/20+1/18-1/21)
=(1/3)×(1+1/2+1/3+.1/18) - (1/4+1/5+1/6+.1/21)
=(1/3)×[(1+1/2+1/3)-(1/19+1/20+1/21)]
잠깐