数列a1,a2,...,an,...有极限A,数列b1,b2,...,bn,...也有极限B
极限乘法运算律：lim(an*bn)=lim(an)*lim(bn)=A*B
证明：
anbn-AB=(an-A)(bn-B)+B(an-A)+A(bn-B)
对任意εN₁时,有|B||an-A|N₂时,有|A||bn-B|N=Max(N₁,N₂)时,有|an-A||bn-B|