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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 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](https://tex.z-dn.net/?f=+frac%7B1%7D%7B+frac%7B1%7D%7B18%7D+-++frac%7B1%7D%7B21%7D++%7D++%3D++frac%7B1%7D%7B+frac%7B7%7D%7B126%7D+-++frac%7B6%7D%7B126%7D++%7D++%3D++%5C++%3D++frac%7B1%7D%7B+frac%7B1%7D%7B126%7D+%7D++%3D+1+div++frac%7B1%7D%7B126%7D++%3D+1+times++frac%7B126%7D%7B1%7D++%3D++%5C++%3D+126)
12.
![(2x + 3y) {}^{2} - 3x( frac{4}{3} x + 4y) = \ = 4 {x}^{2} + 12xy + 9 {y}^{2} - 4 {x}^{2} - 12xy = \ = 9y {}^{2} (2x + 3y) {}^{2} - 3x( frac{4}{3} x + 4y) = \ = 4 {x}^{2} + 12xy + 9 {y}^{2} - 4 {x}^{2} - 12xy = \ = 9y {}^{2}](https://tex.z-dn.net/?f=%282x+%2B+3y%29+%7B%7D%5E%7B2%7D++-+3x%28+frac%7B4%7D%7B3%7D+x+%2B+4y%29+%3D++%5C++%3D+4+%7Bx%7D%5E%7B2%7D++%2B+12xy+%2B+9+%7By%7D%5E%7B2%7D++-+4+%7Bx%7D%5E%7B2%7D++-+12xy+%3D++%5C++%3D+9y+%7B%7D%5E%7B2%7D+)
подставим у=√3
![9 {y}^{2} = 9 times (sqrt{3} {)}^{2} = 9 times 3 = 27 9 {y}^{2} = 9 times (sqrt{3} {)}^{2} = 9 times 3 = 27](https://tex.z-dn.net/?f=9+%7By%7D%5E%7B2%7D++%3D+9+times++%28sqrt%7B3%7D++%7B%29%7D%5E%7B2%7D++%3D+9+times+3+%3D+27)
12.
подставим у=√3
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