4.-(a+1)²+9(a-2)²

-(a+1)²+9(a-2)²
=9(a-2)²-(a+1)²
=[3(a-2)+(a+1)][3(a-2)-(a+1)]
=[3a-6+a+1][3a-6-a-1]
=(4a-5)(2a-7)看不懂第二步-(a+1)²+9(a-2)²=9(a-2)²-(a+1)² 加法交换律=[3(a-2)+(a+1)][3(a-2)-(a+1)] 平方差公式=[3a-6+a+1][3a-6-a-1]去括号=(4a-5)(2a-7)括号内合并同类项6.1-84x的4次方系数是81还是84啊？又打错了。6题1-81x的4次方1-81x的4次方=1²-(9x²)²=(1+9x²)(1-9x²)=(1+9x²)(1+3x)(1-3x)