1/4+1/28+1/70+1/130+1/208=

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1/4+1/28+1/70+1/130+1/208=1/4+1/28+1/70+1/130+1/208=1/4+1/28+1/70+1/130+1/208==1/(1*4)+1/(4*7)+1/(7*

1/4+1/28+1/70+1/130+1/208=
1/4+1/28+1/70+1/130+1/208=

1/4+1/28+1/70+1/130+1/208=
=1/(1*4)+1/(4*7)+1/(7*10)+1/(10+13)+1/(13*16)
=1/3【(1-1/4)+(1/4‐1/7)+(1/7‐1/10)+(1/10-1/13)+(1/13-1/16)】
=1/3(1-1/16)
= 5/16

5/16

自己算吧

1/4+1/28+1/70+1/130+1/208
= 1/4+1/3(1/4-1/7+1/7-1/10+1/10-1/13+1/13-1/16)
=1/4+1/3*(4/16-1/16)
=1/4+1/16=5/16

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