Giải phương trình: x^2 - 11x + 30 = 0

1) x² -11x + 30 = 0

<=> x² -5x - 6x + 30 = 0

<=> x(x - 5) - 6(x - 5) = 0

<=> (x - 5)(x - 6) = 0

<=> x = 5 hoặc x = 6

2) x² - 10x + 21 = 0

<=> x² - 7x - 3x + 21 = 0

<=> x(x - 7) - 3(x - 7) = 0

<=> (x - 7)(x - 3) = 0

<=> x = 7 hoặc x = 3

3) x² - 12x + 27 = 0

<=> x² - 9x - 3x + 27 = 0

<=> x(x - 9) - 3(x - 9) = 0

<=> (x - 9)(x - 3) = 0

<=> x = 9 hoặc x = 3

4) 5x² - 17x + 12 = 0

<=> 5x² - 5x - 12x + 12 = 0

<=> 5x(x - 1) - 12(x - 1) = 0

<=> (x - 1)(5x - 12) = 0

<=> x = 1 hoặc x = 12/5

5) 3x² - 19x - 22 = 0

<=> 3x² - 22x + 3x - 22 = 0

<=> x(3x - 22) + (3x - 22) = 0

<=> (3x - 22)(x +1) = 0

<=> x = 22/3 hoặc x = - 1

6) x² - (1 + √2)x + √2 = 0

<=> x² - x - √2 x + √2 = 0

<=> x(x - 1) - √2(x - 1) = 0

<=> (x - 1)(x - √2) = 0

<=> x = 1 hoặc x = √2

7) x² - 14x + 33 = 0

<=> x² - 11x - 3x + 33 = 0

<=> x(x - 11) + 3(x - 11) = 0

<=> (x - 11)(x +3) = 0

<=> x = 11 hoặc x = - 3

41) x² - 16x + 84 = 0

<=> x² - 16x + 64 +20 = 0

<=> (x - 8)² +20 = 0 (vô lý)

42) x² + 2x - 8 = 0

<=> x² + 4x - 2x - 8 = 0

<=> x(x + 4) -2(x + 4) =0

<=> (x + 4)(x - 2) = 0

<=> x = -4 hoặc x = 2

43) 5x² + 8x - 4 = 0

<=> 5x² + 10x - 2x - 4 = 0

<=> 5x(x + 2) -2(x + 2) =0

<=> (x + 2)(5x - 2) = 0

<=> x = -2 hoặc x = 2/5