Question

SISTEMA DE EQUAÇOES DO 1 GRAU 7X+Y=42 3X-Y=8

Answer 1

primeiro vc deve isolar uma das icognitas.

7x+y = 42 ==> y= 42-7x

3x-y= 8 ==> y= 3x-8

principio reflexivo da igualdade

y=y

42-7x = 3x-8

42+8 = 3x+7x

50 = 10x

50/10 = x

5 = x

Answer 2

Por Comparação:

$\left \{ {{7x+y=42} \atop {3x-y=8}} \right. \\\\\\7x+y=42\to 7x=-y+42\to \frac{-y+42}{7} \\\\3x-y=8\to 3x=y+8\to x= \frac{y+8}{3} \\\\\\\\ \frac{-y+42}{7} = \frac{y+8}{3} \to 3(-y+42)=7(y+8)\to \\\\ -3y-126=7y+56\to -3y-7y=56-126\to -10y=-70\to\\\\ y= \frac{-70}{-10} \to y=7$


$x= \frac{-y+42}{7}\to x= \frac{-7+42}{7}\to x= \frac{35}{7}\to x=5\\\\\\ x = \frac{y+8}{3} \to x = \frac{7+8}{3} \to x = \frac{15}{3} \to x=5\\\\\\\\ S=(5;7)$