1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?

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1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?1X2/5+2X3/5+3X4/

1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?
1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?

1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?
1X2/5+2X3/5+3X4/5+4X5/5.+48X49/5+49X50/5=?
=5(1-1/2)+5(1/2 -1/3)+5(1/3-1/4)+5(1/4-1/5)+.+5(1/48-1/49)+5(1/49-1/50)
=5(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+.+1/48-1/49+1/49-1/50)
=5(1-1/50)
=5*(49/50)
=49/10
=4.9

课一把上述式子看成一个数列,则它的通项公式是An=n(n+1)/5=n^2/5+n/5,所以就可把它看成一个等比数列n^2/5和一个等差数列n/5之和
思路有了,剩下的就是你的计算了

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