第二问:因为an=5n-3,
而bn=2/[an*a(n+1)],
所以,bn=2/[(5n-3)*(5n+2)]
裂项,bn=2/5乘以[1/(5n-3) - 1/(5n+2)]
所以Sn = 2/5乘以[1/2-1/(5n+2)]
依题意Sn