解下列方程: (1)2.5x÷3=6 (2)x+3

解下列方程: (1)2.5x÷3=6 (2)x+34x=78 (3)2x-13=17.

(1)2.5x÷3=6,
     2.5x÷3×3=6×3,
           2.5x=18,
      2.5x÷2.5=18÷2.5,
              x=7.2;

(2)x+
3
4
x=
7
8

      
7
4
x=
7
8

  
7
4
7
4
=
7
8
÷
7
4

         x=
1
2


(3)2x-
1
3
=
1
7

  2x-
1
3
+
1
3
=
1
7
+
1
3

        2x=
10
21

     2x÷2=
10
21
÷2,
         x=
5
21
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