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You are obviously not watching our free lectures (they go through everything you need to pass F9).
The plus or minus is simply to indicate whether it is a fall or a rise in the variable that we are considering will cause a problem.
In this example, we are only worried if the inflow decreases (not if it increases – in that case things only get better). To show that it is only a decrease that causes a problem for us we put a minus sign.
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