5(y-4)-7(7-y)-9=12-3(9-y)

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5(y-4)-7(7-y)-9=12-3(9-y)5(y-4)-7(7-y)-9=12-3(9-y)5(y-4)-7(7-y)-9=12-3(9-y)原方程可化为:5y--20--(49--7y)--

5(y-4)-7(7-y)-9=12-3(9-y)
5(y-4)-7(7-y)-9=12-3(9-y)

5(y-4)-7(7-y)-9=12-3(9-y)
原方程可化为:
5 y -- 20 -- ( 49 -- 7y ) -- 9 = 12 -- ( 27 -- 3y )
去括号得 :5 y -- 20 -- 49 + 7y -- 9 = 12 -- 27 + 3y
移项得:5y + 7y -- 3y = 12 -- 27 + 9 + 20 + 49
9y = 63
y = 7

5y-20-49+7y=12-27+3y
9y=54
y=6