x²-ax+a-1=x²-1-a(x-1)=(x+1)(x-1)-a(x-1)=(x-1)(x-a+1)
x²-ax+a-1=0x^2-[(a-1)+1]x+1*(a-1)=0[x-(a-1)](x-1)=0(x-a+1)(x-1)=0
