Question

giải pt theo pp phân tích thành nt

2x^2 -5x-15=0

Answer 1

`2x^2 - 5x - 15 =0`

`2x^2 - 5x + 6x  - 15 =0`

`(2x ^ 2 - 5x) + 6x - 15 = 0`

`x(2x - 5) + 3(2x - 5) = 0`

`(2x - 5)(x + 3) = 0`

`2x - 5 = 0`

`x + 3 = 0`

Answer 2

`2x^2 - 5x - 15 = 0`

`<=> 2(x^2 - 5/2 x - 15/2) = 0`

`<=> x^2 - 5/2 x - 15/2 = 0`

`<=> x^2 - 2x.5/4 + (5/4)^2 - 145/16 = 0`

`<=> (x-5/4)^2 - (sqrt{145}/4)^2 = 0`

`<=> (x - 5/4 + sqrt{145}/4)(x - 5/4 - sqrt{145}/4) = 0`

`<=> (x+(-5+sqrt{145})/4)(x - (5+sqrt{145})/4) = 0`

`<=>` \(\left[{}\begin{matrix}x=\dfrac{5-\sqrt{145}}{4}\\x=\dfrac{5+\sqrt{145}}{4}\end{matrix}\right.\)

Vậy....