y已知F(X)=3X^2-2X,数列AN的前N项和为SN,点(N,SN)均在函数y=f(x)上,求AN BN=3/AN*A(N-1)使BN前N项和TN<M/20求最小正整数M
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y已知F(X)=3X^2-2X,数列AN的前N项和为SN,点(N,SN)均在函数y=f(x)上,求AN BN=3/AN*A(N-1)使BN前N项和TN<M/20求最小正整数M
y已知F(X)=3X^2-2X,数列AN的前N项和为SN,点(N,SN)均在函数y=f(x)上,求AN BN=3/AN*A(N-1)使BN前N项和TN
<M/20求最小正整数M
y已知F(X)=3X^2-2X,数列AN的前N项和为SN,点(N,SN)均在函数y=f(x)上,求AN BN=3/AN*A(N-1)使BN前N项和TN<M/20求最小正整数M
x=n f(x)=Sn代入函数方程:
Sn=3n²-2n
n=1时,S1=a1=3-2=1
n≥2时,Sn=3n²-2n S(n-1)=3(n-1)²-2(n-1)
Sn-S(n-1)=an=3n²-2n-3(n-1)²+2(n-1)=6n-5
n=1时,a1=6-5=1,同样满足
数列{an}的通项公式为an=6n-5.
bn=3/[ana(n+1)]=3/[(6n-5)(6n+1)]=(1/2)[1/(6n-5)-1/(6n+1)]
Tn=b1+b2+...+bn
=(1/2)[1-1/7+1/7-1/13+...+1/(6n-5)-1/(6n+1)]
=(1/2)[1-1/(6n+1)]
=3/(6n+1)
令Tn60/7,又M为正整数,M≥9,M的最小值为9.