1、用累加法求an=an-1+f(n)型通项
2、用累积法求an= f(n)an-1型通项
3、用待定系数法求an=Aan-1+B型数列通项
4、通过Sn求弯虚山an
5、取倒数转化为等差数列
6、构造函数模型转化为等比数列
7、数学归纳法
普遍的方法举例:
(1)数列{an}满足a1=1且an=an-1+3n-2(n≥2),求an
解:由an=an-1+3n-2知an-an-1=3n-2,记f(埋中n)=3n-2= an-an-1
则an= (an-an-1)+(an-1-an-2)+(an-2-an-3)+…(a2-a1)+a1
=f(n)+ f(n-1)+ f(n-2)+…f(2)+ a1
=(3n-2)+[3(n-1)-2]+ [3(n-2)-2]+ …+(3×2-2)+1
=3[n+(n-1)+(n-2)+…+2]-2(n-1)+1
=3×2((n+2)(n-1))-2n+3=2(3n2-n)
(2)数列{an}满足a1=1且an=an-1+2n(1)(n≥2),求an。
解:由an=an-1+2n(1)知an-an-1=2n(1),记f(n)=2n(1)= an-an-1
则an=(an-an-1)+(an-1-an-2)+(an-2-an-3)+…(a2-a1)+a1
=f(n)+ f(誉轮n-1)+ f(n-2)+…f(2)+ a1
=2n(1)+2n-1(1)+2n-2(1)+…+22(1)+1=2(1)-2n(1)
(3)已知数列{an}满足a1=1且an=n(2(n-1))an—1(n≥2),求an
解:(1)由条件 an—1(an)=n(2(n-1)),记f(n)=n(2(n-1))
an= an—1(an)· an—2(an-1)·… a1(a2)·a1=f(n)f(n-1)f(n-2)…f(2)f(2)a1
=n(2(n-1))·n-1(2(n-2))·n-2(2(n-3))·…3(2×2)·2(2×1)·1=n(2n-1)