原式=3aaa+3aa+9aa+9a-15a-15+3=3aa(a+1)+9a(a+1)-15(a+1)+3=3(a+1)(aa+3a-5)+3=3(a+1)(aa+a+2a+2-7)+3=3(a+1)[a(a+1)+2(a+1)-7]+3=3(a+1)[(a+1)(a+1+1)-7]+3=3(a+1)[(a+1)(a+1)+(a+1)-7]+3...