Question

用累加法和累乘法求数列通项公式

用累加法和累乘法求数列通项公式求前两问，第一问用累加，第二问用累乘

Answer 1

，

追答

Answer 2

(1)
a1=3
a(n+1) = an + 1/[n(n+1)]
a(n+1) -an = 1/n - 1/(n+1)
an - a(n-1) = 1/(n-1) - 1/n
an - a1 = [1/(n-1) - 1/n] +[1/(n-2) - 1/(n-1)] +...+[1/(2-1) - 1/2]
a1 -3 = 1 - 1/n
a1 = 4 - 1/n
(2)
a1 =1
a(n+1) =2^n .an
a(n+1)/an = 2^n
an/a(n-1) = 2^(n-1)
an/a1 = 2^[1+2+...+(n-1) ]

an/1 = 2^[n(n-1)/2]
an =2^[n(n-1)/2]

追问

麻烦能用图片形式吗，符号看不懂