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D A U N = A N + ∑ i = 1 N = A N − 1 ∗ R i DAU_N=A_N+\sum_{i=1}^N=A_{N-1}*R_i DAUN=AN+∑i=1N=AN−1∗Ri
$DAU_N=A_N+\sum_{i=1}^N=A_{N-1}*R_i$
然后自己找规律改就行