Giải pt sau

90/x - 90/ 90 - x = 9/20

`90/x-90/(90-x)=9/20` `(ĐK:x\ne0;x\ne90)`

`<=>` `(90.20(90-x))/(20x(90-x))-(90.20x)/(20x(90-x))=(9.x(90-x))/(20x(90-x))`

`=>` `1800(90-x)-1800x=9x(90-x)`

`<=>` `162000-1800x-1800x=810x-9x^2`

`<=>` `9x^2-180-x-1800x-810x+16200=0`

`<=>` `9x^2-4410x+16200=0`

`<=>` `x^2-490x+18000=0`

`Delta'=(-245)^2-1.18000=42025>0`

`=>` Phương trình có hai nghiệm phân biệt

`x_{1}=(-b'+sqrt{Delta})/a=(-(-245)+sqrt{42025})/1=450`

`x_{1}=(-b'-sqrt{Delta})/a=(-(-245)-sqrt{42025})/1=40`

Vậy `S={40;450}`