Tìm x, biết: x(х - 5) - 4х + 20 = 0

49. Tìm x, biết: a) x(х- 5)- 4х + 20= 0%3 b) x(x + 6) – 7x - 42 0; c) x - 5x + x - 5 03; = d) x'-2x + 10x2 - 20x = 0.