Find the maximum area of a rectangle that can be inscribed in a unit circle.
Let half the width of the rectangle = w
Then half the length of the rectangle = √[ 1 - w2 ]
And let the area of the rectangle = A
\(A\ =\ 4w\sqrt{1-w^2}\\~\\ \frac{dA}{dw}\ =\ 4\frac{d}{dw}\big(w\sqrt{1-w^2}\,\big)\\~\\ \frac{dA}{dw}\ =\ 4\Big[(w)(\frac12)(1-w^2)^{-\frac12}(-2w)+(\sqrt{1-w^2})(1)\Big]\\~\\ \frac{dA}{dw}\ =\ 4\Big[\frac{-w^2}{\sqrt{1-w^2}}+\sqrt{1-w^2}\Big]\\~\\ \frac{dA}{dw}\ =\ 4\Big[\frac{-w^2}{\sqrt{1-w^2}}+\frac{1-w^2}{\sqrt{1-w^2}}\Big]\\~\\ \frac{dA}{dw}\ =\ \frac{4-8w^2}{\sqrt{1-w^2}}\)
Now let's find what value of w makes \(\frac{dA}{dw}\) be 0 .
\(0\ =\ \frac{4-8w^2}{\sqrt{1-w^2}}\\~\\ 0\ =\ 4-8w^2\qquad\text{and}\qquad\sqrt{1-w^2}\ \neq\ 0\qquad\text{that is}\qquad w\ \neq\ \pm1\\~\\ 8w^2\ =\ 4\\~\\ w^2\ =\ \frac12\\~\\ w\ =\ \sqrt{\frac12}\qquad\text{or}\qquad w\ =\ -\sqrt{\frac12} \)
We know half the width of the rectangle can't be negative,
and by looking at a graph https://www.desmos.com/calculator/txezockcml
we can confirm that the maximum value of A occurs when \(w=\sqrt{\frac12}\) .
When \(w=\sqrt{\frac12}\) ,
\(A\ =\ 4w\sqrt{1-w^2}\ =\ 4\sqrt{\frac12}\sqrt{1-\frac12}\ =\ 4\sqrt{\frac12}\sqrt{\frac12}\ =\ 4\cdot\frac12\ =\ 2\)
The maximum area is 2 sq units.
Let half the width of the rectangle = w
Then half the length of the rectangle = √[ 1 - w2 ]
And let the area of the rectangle = A
\(A\ =\ 4w\sqrt{1-w^2}\\~\\ \frac{dA}{dw}\ =\ 4\frac{d}{dw}\big(w\sqrt{1-w^2}\,\big)\\~\\ \frac{dA}{dw}\ =\ 4\Big[(w)(\frac12)(1-w^2)^{-\frac12}(-2w)+(\sqrt{1-w^2})(1)\Big]\\~\\ \frac{dA}{dw}\ =\ 4\Big[\frac{-w^2}{\sqrt{1-w^2}}+\sqrt{1-w^2}\Big]\\~\\ \frac{dA}{dw}\ =\ 4\Big[\frac{-w^2}{\sqrt{1-w^2}}+\frac{1-w^2}{\sqrt{1-w^2}}\Big]\\~\\ \frac{dA}{dw}\ =\ \frac{4-8w^2}{\sqrt{1-w^2}}\)
Now let's find what value of w makes \(\frac{dA}{dw}\) be 0 .
\(0\ =\ \frac{4-8w^2}{\sqrt{1-w^2}}\\~\\ 0\ =\ 4-8w^2\qquad\text{and}\qquad\sqrt{1-w^2}\ \neq\ 0\qquad\text{that is}\qquad w\ \neq\ \pm1\\~\\ 8w^2\ =\ 4\\~\\ w^2\ =\ \frac12\\~\\ w\ =\ \sqrt{\frac12}\qquad\text{or}\qquad w\ =\ -\sqrt{\frac12} \)
We know half the width of the rectangle can't be negative,
and by looking at a graph https://www.desmos.com/calculator/txezockcml
we can confirm that the maximum value of A occurs when \(w=\sqrt{\frac12}\) .
When \(w=\sqrt{\frac12}\) ,
\(A\ =\ 4w\sqrt{1-w^2}\ =\ 4\sqrt{\frac12}\sqrt{1-\frac12}\ =\ 4\sqrt{\frac12}\sqrt{\frac12}\ =\ 4\cdot\frac12\ =\ 2\)
The maximum area is 2 sq units.