x的3次方-2x[2分之1x的平方-3(3分之1x-1)],其中x=-2分之1
=x³-x³+6x(3分之1x-1)
=2x²-6x
=2*(-1/2)²-6*(-1/2)
=1/2+3
=7/2
=3.5