3x+5y=12 |inmultit cu 5
5x-4y=-8 |inm. cu -3
_______________
15x+25y=60
-15x+12y=24
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adunand cele 2 ecuatii obtinem
37y=84
y=84/37
3x+5.84/37=12
111x+420=74
111x=74-420=-346
x=-346/111
b)
4(-2y)+5y-18=0
-8y+5y=18
-3y=18
y=-6
x=-2y=12
c)
x+y=7
-x+7y=9
adunand ecuatiile
x-x+y+7y=16
8y=16
y=2
x=7-y=7-2=5