∵ac=-1 ab=-1/2∴ac/ab=c/b=(-1)/（-1/2）=2c=2b b=-2a c=-4a(2a-b+c)²-1/4(2a+b-c)²=(2a-b+c-a-1/2b+1/2c)(2a-b+c+a+1/2b-1/2c)=(a-3/2b+3/2c)(3a-1/2b-1/2c)=(a+3a-6a)(3a+a+2a)=(-3a)/(6a)=-1/2
