an是等差数列2s2+s3+s5=20求an的通项公式
a1 S1 = a1
a2 = a1 + d S2 = 2a1 + d
a3 = a1 + 2d S3 = 3a1 + 3d
a4 = a1 + 3d S4 = 4a1 + 6d
a5 = a1 + 4d S3 = 5a1 + 10d ;
2s2 + s3 + s5 = 4a1 + 2d + 3a1 + 3d + 5a1 + 10d = 12a1 + 15d = 20;
d = ( 20 - 12a1 )/15 = 4/3 - 4a1/5;
故,an = a1 + (n-1)( 4/3 - 4a1/5 )
= a1 + 4n/3 - 4na1/5 - 4/3 + 4a1/5
= (a1/5)( 9 - 4n ) + (4/3)( n - 1 );
若 a1 = 1,则 an = ( 8n + 7 )/15 。
题目缺少条件
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