Question

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2x-3y=7
5x-4y=7

Answer 1

2x-3y=7 . (-4)
5x-4y=7 . (3)

- 8x + 12y = - 28
15x - 12y = 21

7x = - 7
x = - 1

2x - 3y = 7
2. (-1) - 3y = 7
- 2 - 3y = 7
- 3y = 7 + 2
- 3y = 9 . (-1)
3y = - 9
y = - 3

S = { - 1 ; - 3 }


Espero ter ajudado.

Answer 2

Por Comparação:

$\left \{ {{2x-3y=7} \atop {5x-4y=7}} \right. \\\\\\ 2x-3y=7\to 2x=7+3y\to x= \frac{7+3y}{2} \\\\5x-4y=7\to 5x=7+4y\to x= \frac{7+4y}{5} \\\\\\\\ \frac{7+3y}{2}= \frac{7+4y}{5}\to 5(7+3y)=2(7+4y)\to 35+15y=14+8y\to \\\\15y-8y=14-35\to 7y=-21\to y= \frac{-21}{7} \to y=-3$



$x= \frac{7+3y}{2}\to x= \frac{7+3*(-3)}{2}\to x= \frac{7+(-9)}{2}\to x= \frac{7-9}{2}\to x= \frac{-2}{2} \to \\x=-1\\\\\\x= \frac{7+4y}{5}\to x= \frac{7+4*(-3)}{5}\to x= \frac{7+(-12)}{5}\to x= \frac{7-12}{5}\to x= \frac{-5}{5} \to \\\\ x=-1\\\\\\\\S= \left \{ -1;-3\left \}$