1×4分之1 + 4×7分之1 + + ...+73×76分之1 求方法

1/(1×4)=(1-1/4)/3即(1×4)分之1=(1-4分之1)除以3
同理可得
1/(1×4)+1/(4×7)+1/(7×10)+……+1/(73×76)
=(1-1/4)/3+(1/4-1/7)/3+(1/7-1/10)/3+……+(1/73-1/76)/3
=[(1-1/4)+(1/4-1/7)+(1/7-1/10)+……+1/73-1/76]/3
=(1-1/76)/3
=(75/76)/3
=25/76
即76分之25