Завдання 7.

1)

[tex]5x - 3y = 18 \\ - 3y = 18 - 5x \\ y = \frac{18 - 5x}{ - 3} \\ y = \frac{5x - 18}{3} [/tex]

2)

[tex]4a = 1 + 6ab \\ 4a - 6ab = 1 \\ a(4 - 6b) = 1 \\ a = \frac{1}{4 - 6b} [/tex]

Завдання 8.

(3x+y)²=(x+3y)²+8(x-y)(x+y)

(3x+y)²=(x+3y)²+8(x²-у²)

(3x+y)²=x²+2*х*3y+(3у)²+8x²-8у²

(3x+y)²=x²+6хy+9у²+8x²-8у²

(3x+y)²=9x²+6хy+у²

(3x+y)²=(3x)²+2*3х*y+у²

(3x+y)²=(3x+у)²