а)

[tex]\displaystyle\bf\\ + \left \{ {{x + y = 5} \atop {2x - y = 4 }} \right. \\ \\ 2x + x = 4 + 5 \\ 3x = 9 \\ x = 9 \div 3 \\ x = 3 \\ \\ 3 + y = 5 \\ y = 5 - 3 \\ y = 2[/tex]

Ответ: ( 3 ; 2 )

б)

[tex]\displaystyle\bf\\\left \{ {{4x - 3y = 17 \: \: | \times 2 } \atop { - 3x + 2y = - 12 \: \: | \times 3 }} \right. \\ \displaystyle\bf\\ + \left \{ {{8x - 6y = 34} \atop { - 9x + 6y = - 36 }} \right. \\ \\ 8x - 9x = 34 - 36 \\ - x = - 2 \\ x = 2 \\ \\ - 3 \times 2 + 2y = - 12 \\ - 6 + 2y = - 12 \\ 2y = - 12 + 6 \\ 2y = - 6 \\ y = - 6 \div 2 \\ y = - 3[/tex]

Ответ: ( 2 ; - 3 )