Question

x^2 + 5x + 6=0                      b,x^4 + 7x^2 - 8=0

Answer 1

`a) x^2 + 5x + 6 = 0`

Ptr có: `\Delta = b^2 - 4ac = 5^2 - 4 . 1 . 6 = 1 > 0`

`=>` Ptr có `2` `n_o` pb

`x_1 = [ -b + \sqrt{\Delta} ] / [ 2a ] = [ -5 + \sqrt{1} ] / 2 = -2`

`x_2 = [ -b - \sqrt{\Delta} ] / [ 2a ] = [ -5 - \sqrt{1} ] / 2 = -3`

Vậy `S = { -2 ; -3 }`

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`b) x^4 + 7x^2 - 8 = 0`

Đặt `x^2 = t` `(t >= 0)`

  `=> t^2 + 7t - 8 = 0`

Ptr có: `\Delta = b^2 - 4ac = 7^2 - 4 . 1 . (-8) = 81 > 0`

`=>` Ptr có `2` `n_o` pb

`t_1 = [ -b + \sqrt{\Delta} ] / [ 2a ] = [ -7 + \sqrt{81} ] / 2 = 1`  (t/m)

`t_2 = [ -b - \sqrt{\Delta} ] / [ 2a ] = [ -7 - \sqrt{81} ] / 2 = -8`  (ko t/m)

   `@ t = 1 => x^2 = 1 <=> x = +-1`

Vậy `S = { +-1 }`