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For reference I started learning differentiation only a few days ago. Question; https://i.stack.imgur.com/pdBRW.jpg

I understand how things work when they are in numbers e.g polynomials, but am completely lost as to how I can apply differentiation to this situation, nor how part 1 is shown. I cant really find any similar questions online either and the mark scheme only says "1 interim step leading to $20-2x^2$". Could anyone help me understand this question?

janes
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  • What's the perimeter of the rectangle expressed in terms of its side lengths and its given value? – Andrew Chin Oct 26 '19 at 19:03
  • @AndrewChin 2x + 2*length = 20 – janes Oct 26 '19 at 19:08
  • The perimeter of a rectangular is Perimeter $P=2x+2l$, where $x$ is the width and $l$ is the length of the rectangular.

    Here we don´t need one side of the length for the rope. Thus the equation is $20m=2x+l$. Solving the equation for l: $l=20-2x$

    And the area of the rectangular is $A=l\cdot x=(20-2x)\cdot x=20x-2x^2$

     – callculus42 Oct 26 '19 at 19:22

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