Question

utilizando o metodo de substituicao calcule os sistema 
a) 3x+y=12
    4x+y=20  

b)  10x-8y= -14
     x+y=4

c) 3x+y=65
    x-2y=10

Answer 1

$\left \{ {{3x+y=12}*(-1) \atop {4x+y=20}} \right. \\ \\ \left \{ {{-3x-y=-12} \atop {4x+y=20}} \right. \\ \\ x= 8 \\ \\ 3x+y=12 \\ 3*8+y=12 \\ 24+y=12 \\ y=12-24 \\ y=-12$