3/4-5/6+7/12-8/15+9/20=3/4+3/12-8/15+9/20= 7/15+9/20
=11/12
4/3-5/6+7/12-8/15+9/20
=(1+3)/(1×3) -(2+3)/(2×3)+(3+4)/(3×4)-(3+5)/(3×5)+(4+5)/(4×5)
=1+1/3 -(1/2+1/3)+(1/3+1/4)-(1/3+1/5)+(1/4+1/5)
=1+1/3-1/2-1/3+1/3+1/4-1/3-1/5+1/4+1/5
=1-1/2+1/4+1/4
=1-1/2+1/2
=1
80/60-50/60+35/60-32/60+27/60=60/60=1
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