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
1×2/5+2×3/5+3×4/5+……+48×49/5+49×50/5=
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