12×(2/15-1/12)-1/4
=12×2/15-12×1/12-1/4
=8/5-1-1/4
=3/5-1/4
=7/20
9/11×(3/5-3/7+2/5-4/7)
=9/11×[(3/5+2/5)-(3/7+4/7)]
=9/11×[1-1]
=9/11×0
=0
12×(2/15-1/12)-1/4
=12×2/15-12×1/12-1/4
=8/5-1-1/4
=3/5-1/4
=7/20
9/11×(3/5-3/7+2/5-4/7)
=9/11×[(3/5+2/5)-(3/7+4/7)]
=9/11×[1-1]
=9/11×0
=0