Plus all the situations, we get the final formula:
For B: From 1 to 8000000 For example : b=60..................
For A: From 1 to 8000000 For example : b=60............ (b+1) (b+2) 7999999 8000000......
Also ， plus all the situations we get the final formula:
Derivative ！！！ a ∈【 1 ， 8000000 】 S’(a)>0 So, the function is always increasing. And for A, at 1 will has the minimum expense. We have written down the function S(a) and S(b),but how to deal with them ?
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