3.

[tex](x + y) {}^{2} = {x}^{2} + 2xy + {y}^{2} [/tex]

4.

[tex] + \left \{ {{x + y = 5} \atop { - x + y = 1 }} \right. \\ \\ 1) \: (x + ( - x)) + (y + y) = 5 + 1 \\ 2 y = 6 \\ y = 6 \div 2 = 3 \\ 2) \: x + y = 5, \: \: y = 3 \\ x + 3 = 5 \\ x = 5 - 3 = 2 \\ otvet \: \: \: (2; \: 3)[/tex]

5.

[tex]y = - x + 5 \\ x = 3 \\ y = y(3) = - 3 + 5 = 2[/tex]

6.

[tex]15 {x}^{2} + 3xy = 3x \times 5x + 3x \times y = 3x(5x + y)[/tex]