Question

Помогите пожалуйста с решением.

Answer 1

1.
$\frac{1}{ \frac{1}{18} - \frac{1}{21} } = \frac{1}{ \frac{7}{126} - \frac{6}{126} } = \\ = \frac{1}{ \frac{1}{126} } = 1 \div \frac{1}{126} = 1 \times \frac{126}{1} = \\ = 126$
12.
$(2x + 3y) {}^{2} - 3x( \frac{4}{3} x + 4y) = \\ = 4 {x}^{2} + 12xy + 9 {y}^{2} - 4 {x}^{2} - 12xy = \\ = 9y {}^{2}$
подставим у=√3
$9 {y}^{2} = 9 \times (\sqrt{3} {)}^{2} = 9 \times 3 = 27$