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S奇-S偶 =an-an-1+an-2-an-3+……+a3-a2+a1 =d+d+d+d+……+a1 =a1+(n-1)d/2 =81-72 =9 Sn=n[a1+(n-1)d/2]=81+72=153 9n=153 n=17
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