4.

a) Ответ: x=5 ; y=3

[tex]\left \{ {{3x - 3y = 6} \atop {2x + 3y = 19}} \right. + \\ 5x = 25 \\ x = 5 \\ \\ 2x + 3y = 19 \\ 2 \times 5 + 3y = 19 \\ 10 + 3y = 19 \\ 3y = 9 \\ y = 3[/tex]

б) Ответ: x=-1 ; y=1

[tex]\left \{ {{4x + 9y = 5 \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: \: } \atop {2x - y = - 3 \: \:| \times 9}} \right. \\ \\\left \{ {{4x + 9y = 5} \atop {18x - 9y = - 27}} \right. + \\ 22x = - 22 \\ x = - 1 \\ \\ 2x - y = - 3 \\ 2 \times ( - 1) - y = - 3 \\ - 2 - y = - 3 \\ - y = - 1 \: \: \: \: \: \: \: \: \: \: y = 1[/tex]

5. Ответ: x=4,7 ; y=2,6

[tex]\left \{ {{x + y = 7.3} \atop {x = y + 2.1}} \right. \\ \\ y + 2.1 + y = 7.3 \\ 2y = 5.2 \\ y = 2.6 \\ \\ x = y + 2.1 \\ x = 2.6 + 2.1 \\ x = 4.7[/tex]