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1+1/4+1/16+1/64+1/256+1/1024

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1+1/4+1/16+1/64+1/256+1/1024
1/18+1/54+1/108+1/180+1/270+1/378+1/504+1/648+1/810+1/990
1+1/4+1/16+1/64+1/256+1/1024
设S=1+1/4+1/16+1/64+1/256+1/1024-------------------①
则S/4= 1/4+1/16+1/64+1/256+1/1024+1/4096--------②
①-②=3S/4=1-1/4096=4095/4096
所以:S=4095/4096*4/3
=1365/1024
1/18+1/54+1/108+1/180+1/270+1/378+1/504+1/648+1/810+1/990
=1/(3*6)+1/(6*9)+1/(9*12)+.+1/(30*33)
=1/3*[(1/3-1/6)+(1/6-1/9)+(1/9-1/12)+.+(1/30-1/33)]
=1/3*(1/3-1/6+1/6-1/9+1/9-1/12+.+1/30-1/33)
=1/3*(1/3-1/33)
=(1/3)*(10/33)
=10/99
类似:1/[a*(a+n)]=(1/n)*[1/a-1/(a+n)]
例子:
1/56=1/(7*8)=1/7-1/8
1/12=1/(2*6)=(1/4)*[1/2-1/6]
.
等等
以上的式子是很重要的,能记住对解这类的题是有帮助的~