2y²-y-1/2=0
y²-y/2-1/4=0
(y²-y/2+1/16)-1/4-1/16=0
(y-1/4)²=5/16
y-1/4=-√5/4或y-1/4=√5/4
得y=(1-√5)/4或y=(1+√5)/4