1/2×1/3+1/3×1/4+1/4×1/5+........+1/9×1/10 =1/2-1/3+1/3-1/4+1/4-1/5+........+1/9-1/10 =2/5
1/2×1/3+1/3×1/4+1/4×1/5+........+1/9×1/10
=1/2-1/3+1/3-1/4+1/4-1/5+........+1/9-1/10
=1/2-1/10
=2/5
1/2×1/3+1/3×1/4+1/4×1/5+........+1/9×1/10
=1/2x3 +1/3x4 + 1/4x5...1/9x10
=1/6+1/12+1/20...1/90
=1/2-1/3+1/3-1/4+1/4....-1/10
=1/2-1/10
=5/10-1/10
=4/10
=2/5
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