a true for all
3x=x+8
-8=x-3x
-8=-2x
4=x
b true for some (eg. -1=x, 1=x)
5x= x+x+x+x+x
5x=5x
0=5x-5x
0=0
c true for none
2(x+5)=2x+5
2x+10=2x+5
10-5= 2x-2x
5=0
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