A(n-4)=A1+(n-5)d=30
S9=4.5*(A1+A9)=18 =>A1+A1+8d=4 =>A1+4d=2
Sn=n*(A1+An)/2=240 =>(2A1+(n-1)d)=480/n
(3)-(2): A1+(n-5)d=480/n-2=30
=>n=15
已知数列{an}为等差数列前n项和为sn且s9=18 ,a(n-4)=30(n>9)若sn=240 求n的值
已知数列{an}为等差数列前n项和为sn且s9=18 ,a(n-4)=30(n>9)若sn=240 求n的值
数学人气:270 ℃时间:2020-03-25 07:27:39
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