已知数列6,9,14,21,30,.其中相邻两项之差成等差数列,求他的通项答:数列6,9,14,21,30,.为a1,a2,a3……an a1=6 a2-a1=3 a3-a2=5 a4-a3=7 ……an-an-1=2n-1 叠加得:an=6+3+5+7+……+2n-1=5+(1+3+5+7+……+2n-1)数列1,3,5,7,……,2n-1是以1为首项,2为公差的等差数列,共有n项 Sn=n(1+2n-1)/2=n×n=n²∴an=5...
已知a,b,c依次成等差数列,它们的和为33,又lg(a-1),lg(b-5),lg(c-6...答:a,b,c依次成等差数列 2b=a+c 1 a+b+c=33 2 lg(a-1),lg(b-5),lg(c-6)2lg(b-5)=lg(a-1)+lg(c-6)lg(b-5)^2=lg(a-1)(c-6)(b-5)^2=(a-1)(c-6) 3 1式代入2式得 3b=33 b=11 a+c=22 -->a=22-c 4 (11-5)^2=(a-1)(c-6)=36 ...
已知数列6,9,14,21,30,...其中相邻两项之差成等差数列,求他的通项答:解:数列6,9,14,21,30,...为a1,a2,a3……an a1=6 a2-a1=3 a3-a2=5 a4-a3=7 ……an-an-1=2n-1 叠加得:an=6+3+5+7+……+2n-1=5+(1+3+5+7+……+2n-1)数列1,3,5,7,……,,2n-1是以1为首项,2为公差的等差数列,共有n项 Sn=n(1+2n-1)/2=n×n=n&...