解方程⑴2x^2-4x+3 √ x^-2x+6=15 ⑵ 3/(x^2-2x-3)=2x^2-4x-1需步骤

(1)2x²-4x+3√(x²-2x+6)=152x²-4x+12+3√(x²-2x+6)-27=02(x²-2x+6)+3√(x²-2x+6)-27=02[√(x²-2x+6)]²+3√(x²-2x+6)-27=0令√(x²-2x+6)=t,√(x²-2x+6)=√[(...3/(x^2-2x-3)=2x^2-4x-1这道如果用换元法解应设t为什么