求和:a 2-3ab+(-3ab+9b 2)=a 2-6ab+9b 2=(a-3b) 2;
求差:a 2-3ab-(-3ab+9b 2)=a 2-9b 2=(a+3b)(a-3b)或9b 2-a 2=(3b+a)(3b-a).