1/10*11+1/11*12+1/12*13.1/19*20

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1/10*11+1/11*12+1/12*13.1/19*201/10*11+1/11*12+1/12*13.1/19*201/10*11+1/11*12+1/12*13.1/19*201/(10*1

1/10*11+1/11*12+1/12*13.1/19*20
1/10*11+1/11*12+1/12*13.1/19*20

1/10*11+1/11*12+1/12*13.1/19*20
1/(10*11)=1/10-1/11
1/10*11+1/11*12+1/12*13.1/19*20
=(1/10-1/11)+(1/11-1/12)+(1/12-1/13)+...+(1/18-1/19)+(1/19-1/20)
=1/10-1/20
=1/20