第4章 4.2

(1)

$x^2 - x - 30 = 0$ $(x-6)(x+5) = 0$ $x = -5, 6$

(2)

$4x^2 + 5x + 1 = 0$ $(4x+1)(x+1) = 0$ $x = -\dfrac{1}{4}, -1$

(3)

$3x^2 + 3x - 1 = 0$ $x = \dfrac{-3 \pm \sqrt{9+12}}{6} = \dfrac{-3 \pm \sqrt{21}}{6}$

(4)

$3x^2 - 10x - 1 = 0$ $x = \dfrac{5 \pm \sqrt{25+3}}{3} = \dfrac{5 \pm 2\sqrt{7}}{3}$

(5)

$x^2 - 0.7x + 0.1 = 0$ $x = \dfrac{0.7 \pm \sqrt{0.49-0.4}}{2} = \dfrac{0.7 \pm \sqrt{0.09}}{2} = \dfrac{0.7 \pm 0.3}{2} = \dfrac{1}{5}, \dfrac{1}{2}$

(6)

$0.3x^2 - 2x - 1.2 = 0$ $x = \dfrac{1 \pm \sqrt{1+0.36}}{0.3} = \dfrac{1 \pm \sqrt{1.36}}{0.3} = \dfrac{10 \pm 2\sqrt{34}}{3}$