Question

高三数学数列第九题

高三数学数列第九题求全过程

Answer 1

an=(n+1)/2 an=a1+(n-1)/2
a1=1 a2=3/2 a3=2 a4=5/2 .......
d=1/2

追问

第二题呢

第二小题

过程哦！

追答

b1=1/2 b2=4/15 b3=1/6 b4=4/35 ........bn=4/(n+1)(n+3)
b1+b2+b3+b4+...+bn
=1/2+4/15+1/6+4/35+...+4/(n+1)(n+3)
=4/2×4+4/3×5+4/4×6+...+4/n（n+2）+4/(n+1)(n+3)
=2×[1/2-1/4+1/3-1/5+1/4-1/6+...1/n-1/(n+2）+1/(n+1)-1/(n+3)]
=2×[1/2-1/(n+2）+1/3-1/(n+3)]
=2×[5/6-(2n+5)/(n+2)(n+3)]
=5/3-2(2n+5)/[(n+2)(n+3)]