`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 }`
