等差数列{an}中,a1=1,an=77,Sn=780,求n与d在等差数列]{an}中,已知a1=2,a11=32,求公差d已知a2=3,d=5,an=103,求n与Sn已知a31=40,S31=2170,求a1与d已知a1=200,S41=4100,求d与a11

Question

等差数列{an}中,a1=1,an=77,Sn=780,求n与d
在等差数列]{an}中,已知a1=2,a11=32,求公差d
已知a2=3,d=5,an=103,求n与Sn
已知a31=40,S31=2170,求a1与d
已知a1=200,S41=4100,求d与a11

Answer 1

1、公差d=(a11-a1)/10=(32-2)/10=32、∵an=a2+(n-2)d∴n=(an-a2)/d+2=(103-3)/5+2=22又∵a1=a2-d=-2∴Sn=(a1+an)n/2=(-2+103)x22/2=11113、∵S31=31(a1+a31)/2=31*(a1+40)/2=2170即31(a1+40)=4340解得：a1=100∴公差...