(1+1/2+1/3+1/4)×(1/2+1/3+1/4)-(1/2+1/3+1/4+1/5)×(1/2+1/3+1/4)
=(1/2+1/3+1/4)×(1+1/2+1/3+1/4 -1/2-1/3-1/4-1/5)
=(1/2+1/3+1/4)×(1-1/5)
=13/12 × 4/5
=13/15
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(1+1/2+1/3+1/4)*(1/2+1/3+1/4)-(1/2+1/3+1/4+1/5)*(1/2+1/3+1/4)
=(1/2+1/3+1/4)*[(1+1/2+1/3+1/4)-(1/2+1/3+1/4+1/5)]
=(1/2+1/3+1/4)*(1-1/5)
=(1/2+1/3+1/4)*4/5
=13/12 * 4/5
=13/15