`a)`

`7x^2-9x+2=0`

Ta có: `7+(-9)+2=0`

`⇒x_1=1`

`x_2=c/a=2/7`

Vậy `x in {1;2/7}`

`b)`

`23x^2-9x-32=0`

Ta có: `23-(-9)+(-32)=0`

`⇒x_1=-1`

`x_2=-c/a=-(-32)/23=32/23`

Vậy `x in {-1;32/23}`

`c)`

`1975x^2+4x-1979=0`

Ta có: `1975+4+(-1979)=0`

`⇒x_1=1`

`x_2=c/a=-1979/1975`

Vậy `x in {1;-1979/1975}`

`d)`

`31,1x^2-50,9x+19,8=0`

Ta có: `31,1+(-50,9)+19,8=0`

`⇒x_1=1`

`x_2=c/a=(19,8)/(31,1)=198/311`

Vậy `x in {1;198/311}`