3x²-｛5x²-【2x-（3x²-3x+x²）】+5x｝ =3x²-5x²+【2x-（3x²-3x+x²）】-5x =3x²-5x²+2x-（3x²-3x+x²）-5x =3x²-5x²+2x-3x²+3x+x²--5x =-6x² x=二分之一时 原式＝－3/2
