`C=(a^4-4a^2+3)/(a^4-12a^2+27)` ĐK: `a \ne +-3; +-\sqrt{3}`
`C=(a^4-3a^2-a^2+3)/(a^4-9a^2-3a^2+27)`
`C=(a^2(a^2-3)-(a^2-3))/(a^2(a^2-9)-3(a^2-9))`
`C=((a^2-3)(a^2-1))/((a^2-9)(a^2-3))`
`C=(a^2-1)/(a^2-9)`
Thay `a=\sqrt{3}-\sqrt{2}` vào C ta có:
`C=((\sqrt{3}-\sqrt{2})^2-1)/((\sqrt{3}-\sqrt{2})^2-9)`
`C=(3-2\sqrt{6}+2-1)/(3-2\sqrt{6}+2-9)`
`C=(4-2\sqrt{6})/(-4-2\sqrt{6})`
`C= (\sqrt{6}-2)/(\sqrt{6}+2)`
`C=(\sqrt{6}-2)^2/((\sqrt{6}+2)(\sqrt{6}-2))`
`C=(6-4\sqrt{6}+4)/(6-4)`
`C= (10-4\sqrt{6})/2`
`C=5-2\sqrt{6}`
Vậy `C=5-2\sqrt{6}` khi `a=\sqrt{3}-\sqrt{2}`
