1/(a1*a2)+1/(a2*a3)+1/(a3*a4)……+1/(an*an+1)=1/(a2-a1)*（1/a1-1/a2）+……+1/(an-an+1)*（1/an-1/an+1）=1/d（1/a1-1/a2+1/a2-1/a3+……+1/an-1/an+1）=（1/d)(1/a1-1/an+1)
等差数列（an)的公差为d>0,a1>0,证明1/(a1*a2)+1/(a2*a3)+1/(a3*a4)……+1/(an
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