Đáp án:
`a, 25x^2 - 12= 2^2 . 5 + (-2)^2`
`<=> 25x^2 - 12= 20 + 4`
`<=> 25x^2 = 24 + 12`
`<=> 25x^2= 36`
`<=> x^2= 36/25`
`<=> x= 6/5`
Vậy `S={6/5}`
`b, (x+4)^2 - (1+x)(x-1)=16`
`<=> x^2 + 8x + 16 - x + 1 - x^2 + x = 16`
`<=> 8x + 17 = 16`
`<=> 8x= -1`
`<=> x= -1/8`
Vậy `S={-1/8}`
`c, (2x-1)^2 + (x+3)^2 -5(7+x)(x-7)=2x`
`<=> 4x^2 - 4x + 1 + x^2 + 6x + 9 - 5(7x- 49 + x^2 - 7x)= 2x`
`<=> 4x^2 - 4x + 1 + x^2 + 6x + 9 - 35x + 245 - 5x^2 + 35x - 2x = 0`
`<=> 255=0` ( vô lí)
`=>` Pt trên vô nghiệm
Vậy `S=∅`
`d, (x-2)^3 - (x-3)(x^2 + 9+ 3x)+ 6(x+1)^2= 98`
`<=> 24x + 25=98`
`<=> 24x = 73`
Vậy `S={73/24}`
