a1=3,an=2n+1,Sn=195,求d,n这是个A.P.
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a1=3,an=2n+1,Sn=195,求d,n这是个A.P.
a1=3,an=2n+1,Sn=195,求d,n
这是个A.P.
a1=3,an=2n+1,Sn=195,求d,n这是个A.P.
a1=3,an=2n+1,则:
a(n-1)=2*(n-1)+1=2n-1
d=an-a(n-1)=(2n+1)-(2n-1)=2
Sn=a1+a2+……+an=1/2(a1+an)=n(n+2)=195,得n=13
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