Maths DSE M!?

(c)
"The running cost at time t is 100/(t + 9) thousand dollars."
running cost 係指 經營費用

而 ci 問 "the total expenditure E (in thousand dollars)"
可以講係求所有嘅 "經營費用"
即係將所有嘅 running cost 加埋一齊
所以 ci 條式可以 ∫ 100/(t + 9) dt



至於 ciii 題目應該係講
During the period of the campaign, manager 建議用 less costly plan 而唔用原本嘅 campaign
咁呢個 plan 就 modelled by the function g(t) = -(t - α)(t - 2α) , α ≤ t ≤ 2α
而 α 係指呢個 plan 嘅 starting time
題目問：為左得到 the maximum total amount of sales increased, 咁應該幾時開始


呢題應首先積左條式，然後再睇番 α 係幾時先會做到 the maximum total amount