已知等差数列{an}的前n项和为Sn,若a10=30,a20=50.(1)求数列{an}的通项公式.(2)若Sn=242,求n
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已知等差数列{an}的前n项和为Sn,若a10=30,a20=50.(1)求数列{an}的通项公式.(2)若Sn=242,求n
已知等差数列{an}的前n项和为Sn,若a10=30,a20=50.(1)求数列{an}的通项公式.(2)若Sn=242,求n
已知等差数列{an}的前n项和为Sn,若a10=30,a20=50.(1)求数列{an}的通项公式.(2)若Sn=242,求n
设公差为d
30=a1+(10-1)d
50=a1+(20-1)d
a1=12,d=2
(1)数列{an}的通项公式为
an=12+2(n-1)
即an=2n+10
(2)若Sn=242
n(12+2n+10)/2=242
n²+11n-242=0
n=11(n=-22不合题意舍去)
b=(50-30)/(20-10)=2;
a1=30-18=12
数列{an}的通项公式=12+2(n-1);
Sn=242=(12+2(n-1)+12)n/2=n(n+11);
n=11
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