Sn=(a+a²+...+a的n次方)-(1+2+...+n),为什么a=0时,Sn=-[(n²+n)/2],a=1时,Sn=n-[n(n+1)]/2=(n-n²)/2a≠0且a≠1时,Sn=[a(1-a的n次方)/1-a]-[(n²+n)/2]
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![Sn=(a+a²+...+a的n次方)-(1+2+...+n),为什么a=0时,Sn=-[(n²+n)/2],a=1时,Sn=n-[n(n+1)]/2=(n-n²)/2a≠0且a≠1时,Sn=[a(1-a的n次方)/1-a]-[(n²+n)/2]](/uploads/image/z/1761850-10-0.jpg?t=Sn%3D%EF%BC%88a%2Ba%26%23178%3B%2B...%2Ba%E7%9A%84n%E6%AC%A1%E6%96%B9%EF%BC%89-%EF%BC%881%2B2%2B...%2Bn%EF%BC%89%2C%E4%B8%BA%E4%BB%80%E4%B9%88a%3D0%E6%97%B6%2CSn%3D-%5B%EF%BC%88n%26%23178%3B%2Bn%EF%BC%89%2F2%5D%2Ca%3D1%E6%97%B6%2CSn%3Dn-%5Bn%EF%BC%88n%2B1%EF%BC%89%5D%2F2%3D%EF%BC%88n-n%26%23178%3B%EF%BC%89%2F2a%E2%89%A00%E4%B8%94a%E2%89%A01%E6%97%B6%2CSn%3D%5Ba%EF%BC%881-a%E7%9A%84n%E6%AC%A1%E6%96%B9%EF%BC%89%2F1-a%5D-%5B%EF%BC%88n%26%23178%3B%2Bn%EF%BC%89%2F2%5D)
Sn=(a+a²+...+a的n次方)-(1+2+...+n),为什么a=0时,Sn=-[(n²+n)/2],a=1时,Sn=n-[n(n+1)]/2=(n-n²)/2a≠0且a≠1时,Sn=[a(1-a的n次方)/1-a]-[(n²+n)/2]
Sn=(a+a²+...+a的n次方)-(1+2+...+n),为什么a=0时,Sn=-[(n²+n)/2],
a=1时,Sn=n-[n(n+1)]/2=(n-n²)/2
a≠0且a≠1时,Sn=[a(1-a的n次方)/1-a]-[(n²+n)/2]
Sn=(a+a²+...+a的n次方)-(1+2+...+n),为什么a=0时,Sn=-[(n²+n)/2],a=1时,Sn=n-[n(n+1)]/2=(n-n²)/2a≠0且a≠1时,Sn=[a(1-a的n次方)/1-a]-[(n²+n)/2]
a=0时 Sn=0-(1+2+3+……+n)
=-[(1+n)*n/2]
=-[(n²+n)/2]
不知道你是不是这个意思..