求:1/2×1/4+1/4×1/6+1/6×1/8.1/28×1/30=?
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求:1/2×1/4+1/4×1/6+1/6×1/8.1/28×1/30=?
求:1/2×1/4+1/4×1/6+1/6×1/8.1/28×1/30=?
求:1/2×1/4+1/4×1/6+1/6×1/8.1/28×1/30=?
1/2×1/4+1/4×1/6+1/6×1/8.1/28×1/30
=1/2*[(1/2-1/4)+(1/4-1/6)+(1/6-1/8)+.+(1/28-1/30)]
=1/2*(1/2-1/30)
=1/2*14/30
=7/30
(1/n)*[1/(n+2)]=[1/n-1/(n+2)]/2
原式=[(1/2-1/4)+(1/4-1/6)+……+(1/28-1/30)]/2
=(1/2-1/30)/2
=14/60
=7/30