`C=(2a-1)/(3a-1)+(5-a)/(3a+1)` (`x\ne+-1/3`)
`C=((2a-1)(3a+1)+(5-a)(3a-1))/((3a-1)(3a+1))`
`C=(6a^2+2a-3a-1+15a-5-3a^2+a)/((3a-1)(3a+1))`
`C=(3a^2+15a-6)/((3a-1)(3a+1))`
`C=(3(a^2+5a-2))/(9a^2-1)`
Có: `10a^2+5a=3`
`<=> a^2+5a-2=1-9a^2`
`-> C=(3(1-9a^2))/(9a^2-1)=-3`
Vậy `C=-3` khi `10a^2+5a=3; 9a^2-1\ne0`