1)

[tex]5x + y = 7 \: \: | \times 4\\ 7x - 4y = - 1 \\ \\ 20x + 4y = 28 \\ 7x - 4y = - 1 \\ \\ 20x + 7x = 28 - 1 \\ 27x = 27 \\ x = 27 \div 27 \\ x = 1 \\ \\ 5 \times 1 + y = 7 \\ 5 + y = 7 \\ y = 7 - 5 \\ y = 2[/tex]

Ответ: ( 1 ; 2 )

3)

[tex]5x - 2y = 16 \: \: | \times 3\\ 8x + 3y = 38 \: \: | \times 2 \\ \\ 15x - 6y = 48 \\ 16x + 6y = 76 \\ \\ 15x + 16x = 48 + 76 \\ 31x = 124 \\ x = 124 \div 31 \\ x = 4 \\ \\ 8 \times 4 + 3y = 38 \\ 32 + 3y = 38 \\ 3y =3 8 - 32 \\ 3y = 6 \\ y = 6 \div 3 \\ y = 2[/tex]

Ответ: ( 4 ; 2 )

5)

[tex]4a + 6b = 9 \: \: | \times 5 \\ 3a - 5b = 2 \: \: | \times 6 \\ \\ 20a + 30b = 45 \\ 18a - 30b = 12 \\ \\ 20a + 18a = 45 + 12 \\ 38a = 57 \\ a = \frac{57}{38} \\ a = \frac{3}{2} \\ a = 1 \frac{1}{2} \\ a = 1.5 \\ \\ 4 \times 1.5 + 6b = 9 \\ 6 + 6b = 9 \\ 6b = 9 - 6 \\ 6b = 3 \\ b = \frac{3}{6} \\ b = \frac{1}{2} \\ b = 0.5[/tex]

Ответ: а = 1,5 ; b = 0,5