先將代數(shù)式去括號(hào)、合并同類項(xiàng)化成最簡(jiǎn)形式,再分析x=2和x=-2代數(shù)式的值不變．原式=10x2-4x2-x+3-5x2+6x-1-5x,
=（10-4-5）x2+（-1+6-5）x+（3-1）,
=x2+2,
因?yàn)閤=2或x=-2時(shí),x2的值均為4,原式的計(jì)算結(jié)果都是6．
所以把“x=-2”錯(cuò)抄成“x=2”,計(jì)算結(jié)果也是正確的
3a+4b+7a-b
=10a+3b
=10*0.5+3*0.8
=5+2.4
=7.4
3b+7b-0.5
=10b-0.5
=10*4-0.4
=39.5