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a) x² -2x -15 = 0 , так как

x₁ *x₂ = - 3 * 5 = - 15

x₁ + x₂ = - 3 + 5 = 2

б) x² + x - 20 = 0 , так как

x₁ *x₂ = 4 * (- 5) = - 20

x₁ + x₂ = 4 - 5 = - 1

в) x² - 6x = 0, так как

x(x - 6) = 0

или x₁ = 0 или x - 6 = 0 ⇒ x₂ = 6

г) $x^{2} -\frac{9}{16}=0$

(x - 3/4)(x + 3/4) = 0

или x - 3/4 = 0 ⇒ x₁ = 3/4 или x + 3/4 = 0 ⇒ x₂ = - 3/4

д) x² - 2x - 2 = 0

е) $x^{2} -2x-1=0$

$x_{1} *x_{2}=(1-\sqrt{2})*\frac{1}{\sqrt{2}-1 } =\frac{1-\sqrt{2} }{-(1-\sqrt{2}) }=-1\\x_{1}+x_{2}=(1-\sqrt{2})+\frac{1}{\sqrt{2}-1 }=(1-\sqrt{2})-\frac{1}{1-\sqrt{2} }=\frac{(1-\sqrt{2})^{2}-1}{1-\sqrt{2} }=\frac{1-2\sqrt{2} +2-1}{1-\sqrt{2} }=\frac{2-2\sqrt{2} }{1-\sqrt{2} }=\frac{2(1-\sqrt{2}) }{1-\sqrt{2} }=2$