x²+2x-1=0
x²+2x+1=2
(x+1)²=2
x+1=±2
x1=1,x2= -3
当x=1时
4-1/2x²-x=4-1/2-1=2.5
当x= -3 时
4-1/2x²-x=4-9/2+3=2.5
∴4-1/2x²-x=2.5