?$解：x-7=11-x$? ?$移項,得x+x=11+7$? ?$合并同類項,得2x=18$? ?$系數(shù)化為1 ,得x=9\ $? ?$ $?
?$解：\ x=\frac{1}4x+6$? ?$去分母,得4x=x+24$? ?$移項,得4x-x=24$? ?$合并同類項,得3x=24$? ?$系數(shù)化為1 ,得x=8$?
解：?$12+7y=-3y+2$? ?$移項，得7y+3y=2-12$? ?$合并同類項，得10y=-10$? ?$系數(shù)化為1 ,得y=-1$? ?$ $?
?$解：\ \frac{1}2m-3=\frac{1}3m-2$? ?$去分母,得3m-18=2m-12$? ?$移項，得3m-2m=-12+18$? ?$合并同類項,得m=6$? ?$ $? ?$解:不正確,應改為:$? ?$y-7=6$? ?$y=6+7$? ?$y=13$? ?$解:將x=5代入方程ax-8=20+a,得\ 5a-8=20+a$? ?$解得，a=7$? ?$∴a的值為7$? ?$解:(1) 2(x+2x)=90$? ?$(2)將x=15代入方程2(x+2x)=90,得$? ?$左邊=2×(15+2×15)=90=右邊$? ?$故x=15是所列方程的解;$? ?$將x=20代入方程2(x+2x)=90 ,得$? ?$左邊=2×(20+2×20)=120≠右邊$? ?$故x=20不是所列方程的解$? ?$(3) 2(y+\frac{1}{2}y)=90$?
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