中一數學題

7[x-3(3-x)]=21
x-9+3x=3
4x=12
x=3

要將x乘開左才能解

7[x-3(3-x)]=21
x-3(3-x)=21over7
3-x=3over x-3
3-x=3overx-1
3-x+1=3over x
4-x=3overx
既然4減去的果等於3除的果，x只會是1。

x=3.....

7[x-3(3-x)]=21
7[x-9-3x]=21
7x-63-21x=21
7x-21x=84
-14x-63=21
-14x=84
x=-6

7[x-3(3-x)]=21
7(x-9+3x)=21
7x-63+21x=21
28x-63=21
28x=21+63
28x=84
28x/28=84/28
x=3

7(x-9+3x) = 21
7(4x-9) = 21
28x-63=21
28x=21+63
28x=84
x=3

7[x-3(3-x)]=21
7(3x-9-2x+3x)=21
3x-9-2x+3x=21/7
6x-2x=3+9
4x=12
x=3

7[x-3(3-x)]=21

7(x-9+3x)=21

4x-9=3

4x=12

x=3

7[x-3(3-x)]=21
7[x-9+3x]=21
7[4x-9]=21
4x-9=3
4x=12
x=3