Đáp án:
`a, 7/3 - x = -2/3`
`⇒ x = 7/3 - -2/3`
`⇒ x = 9/3 = 3`
`b, -2/3 x + 1,2 = (-1/2)^2`
`⇒ -2/3 x + 12/10 = ((-1)^2)/2^2`
`⇒ -2/3 x + 6/5 = 1/4`
`⇒ -2/3 x = 1/4 - 6/5`
`⇒ -2/3 x = -19/20`
`⇒ x = -19/20 : -2/3`
`⇒ x = 57/40`
`c, (x - 3)/(x + 5) = 5/7`
`⇔ 7(x - 3) = 5(x + 5)`
`⇔ 7x - 21 = 5x + 25`
`⇔ 7x - 21 - 5x - 25 = 0`
`⇔ 2x - 46 = 0`
`⇔ 2x = 0 + 46 = 46`
`⇒ x = 46/2`
`⇒ x = 23`
`d, (x + 4)/20 = 5/ (x +7)`
`⇔ (x + 4)(x + 7) = 20 . 5`
`⇔ x.x + x.7 + 4.x + 4.7 = 100`
`⇔ x^2 + 7x + 4x + 28 = 100`
`⇔ x^2 + 11x + 28 = 100`
`⇔ x^2 + 11x + 28 - 100 = 0`
`⇔ x^2 + 11x - 72 =0`
`⇒ [x^2 + 2.x.11/2 + (11/2)^2] - (11/2)^2 - 72 = 0`
`⇒ (x + 11/2)^2 - 409/4 = 0`
`⇒ (x + 11/2)^2 = 409/4`
`⇒ (x + 11/2)^2 =` ($\sqrt{\frac{409}{4}}$ )^2`
`⇒ x + 11/2 =` $\sqrt{\frac{409}{4}}$
`⇒ x =` $\sqrt{\frac{409}{4}}$ `- 11/2`
`⇒ x = (-11 +` $\sqrt{\frac{409}{4}}$)/2`
