Question

设等差数列{a n }的公差为d（d＞0），且满足：a 2 •a 5 =55，a 4 +a 6 =22．

Answer 1

（Ⅰ） ( a 1 +d)( a 1 +4d)=55 2 a 1 +8d=22 解得a 1 =3，d=2∴a n =3+（n-1）×2=2n+1（Ⅱ）当n≥2时，b n =a n -a n-1 =2，b 1 =3∴c n =b n •2 n S n =3×2+2×...