Răspuns:
[tex](a + 3)( a + 4) = a \times a + 3 \times 4 + 3 \times a + a \times 4 = {a}^{2} + 12+ 3a + 4a = {a}^{2} + 7a + 12[/tex]
[tex](x + 2)(3x + 1) = x \times 3x + x \times 1 + 2 \times 3x + 2 \times 1 = 3 {x}^{2} + x + 6x + 2 = 3 {x}^{2} + 7x + 2[/tex]
[tex](m + 5n)(5m + n) = m \times 5m + m \times n + 5n \times 5m + 5n \times n = 5 {m}^{2} + mn + 25mn + 5 {n}^{2} = 5 {m}^{2} + 5 {n}^{2} + 26mn[/tex]
[tex]2(a + 3b)(b + 2a) = 2(a \times b + a \times 2a + 3b \times b + 3b \times 2a) = 2 \times (ab + 2 {a}^{2} + 3 {b}^{2} + 6ab) = 2(2 {a}^{2} + 3 {b}^{2} + 7ab) = 4 {a}^{2} + 6 {b}^{2} + 14ab[/tex]
[tex](4 + a)(a + 3) - 7a - 12 = (4 \times a + 4 \times 3 + a \times a + a \times 3) - 7a - 12 = (4a + 12 + {a}^{2} + 3a) - 7a - 12= ( {a}^{2} + 7a + 12) - 7a - 12 = {a}^{2} + 7a + 12 - 7a - 12 = {a}^{2} [/tex]
