Derivatives are just slopes. If a line is y=3x then it's derivative or slope is 3. If a line is y=x^2 then the slope can be modeled as 2x for any x value you plug in. Note that if you have a graph that has a slope that increases then decreased then you have a maximum. Notice the vertex of y=-x^2 5: it has a maximum at x=0. Note that it's slope changes from positive to negative at x=0, this tells you that you have a maximum. This means that when the derivative or slope is zero, you usually have a max or min (few exceptions) so for this optimization problem I have posted, you need come up with an equation to model the expressions and take the derivative of that equation and set it equal to zero to find either a max or minimum which will then make it very easy to find your answer.
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