(1)2/3X-1/5X=7+2/7
7/15X=51/7
X=765/49
(2)(80-9.8)×0.6-2.1
=70.2×0.6-2.1
=(70+0.2)×0.6－2.1
=42+0.12-2.1
=40.02
(3)7/8×[6/7-(1/21+3/7)]
=7/8×[(6/7－3/7)-1/21]
=7/8×(3/7-1/21)
=7/8×8/21
=1/3
(4)25²-12.5²×3.14/2
=25²－(25/2)²×3.14/2
=25²×(1-3.14/8)
=625×243/400
=6075/16
这是我在静心思考后得出的结论,
如果不能请追问,我会尽全力帮您解决的~
如果您有所不满愿意,请谅解~二分之一乘二十分之十一加二分之一乘四十九分之一==1/2×11/20＋1/2×1/49 =1/2×(11/20＋1/49) =1/2×559/980 =559/1960