等差数列中前m项和是m
等差数列中,前m项和是m/n,前n项和是n/m,m不等于n,则前m n项的和是
(Sm)/m-(Sn)/n =[ma1+m(m-1)/2*d]/m-[na1+n(n-1)/2*d]/n =[a1+(m-1)/2*d]-[a1+(n-1)/2*d]=(m-n)d/2 又(Sm)/m-(Sn)/n=(m/n)/m-(n/m)/n=1/n-1/m=(m-n)/mn --->(m-n)d/2=(m-n)/mn[m<>n--->m-n>mnd=2 S(m+n)=a1+a2+......+am+a(m+1)+a(m+2)+......+a(m+n) =Sm+[(a1+md)+(a2+md)+......+(an+md)] [注:a(m+k)=a1+(m+k-1)d=a1+(m-1)d+md=ak+md] =Sm+Sn+nmd =m/n+n/m+2 =(m+n)^2/(mn)
Sm=m/n,Sn=n/m,m≠n, Sm/m-Sn/n=[(a1+am)/2-a1+an)/2}=(am-an)/2=(m-n)d/2= =1/n-1/m=(m-n)/(nm), 所以nmd=2 S(m+ n)=Sm+Sn+nmd=2+m/n+n/m。
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