Question

数学高手进啊~关于数列问题,跪求高分

Answer 1

a5=a3+2d 14=8+2d
d=3

a3=a1+2d a1=2

an=2+3(n-1)=3n-1

bn=1/an

b1=1/2,b2=1/5,b3=1/8,....

4.
(1)
原式

=1/2-1/3+1/3-1/4+1/4-1/5+...1/2n-1/(2n+1)

=1/2-1/(2n+1)

=(2n-1)/2(2n+1)

(2)
原式
=1/2×[1-1/3+1/2-1/4+1/3-1/5+...1/n-

1/(n+2)]

=1/2[1+1/2-1/(n-1)-1/(n-2)]

=3/8-1/2(n-1)-1/2(n-2)

Answer 2

先占个坑。

6.(1)d=3,an=3n-1
(2)bn=1/(3n-1)

4.(1)原式=1/2-1/3+1/3-1/4+1/4-1/5+...1/2n-1/(2n+1)
=1/2-1/(2n+1)
=(2n-1)/2(2n+1)
(2)原式=1/2×[1-1/3+1/2-1/4+1/3-1/5+...1/n-1/(n+2)]
=1/2[1+1/2-1/(n+1)-1/(n+2)]
=3/8-1/2(n+1)-1/2(n+2)

Answer 3

1 an=3n-1

Answer 4

a5=a3+2d 14=8+2d
d=3

a3=a1+2d a1=2

an=2+3(n-1)=3n-1

bn=1/an

b1=1/2,b2=1/5,b3=1/8,....

4.
(1)
原式

=1/2-1/3+1/3-1/4+1/4-1/5+...1/2n-1/(2n+1)

=1/2-1/(2n+1)

=(2n-1)/2(2n+1)

(2)
原式
=1/2×[1-1/3+1/2-1/4+1/3-1/5+...1/n-

1/(n+2)]

=1/2[1+1/2-1/(n-1)-1/(n-2)]

=3/8-1/2(n-1)-1/2(n-2)

Answer 5

6.
(1)
a3=a1+2d=8;
a5=a1+4d=14;
解关于a1，d的方程组可得到：
a1=2,d=3.
所以：
an=a1+(n-1)d=2+(n-1)3=3n-1.