MHB Dividing quarter of an ellipse

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To divide a quarter of an ellipse into two equal halves, one can start with the ellipse in standard form and convert it to polar coordinates. The area can be calculated using integrals, leading to the conclusion that the angle for division, β, is given by β = tan^(-1)(b/a). An alternative method involves stretching the ellipse into a circle, using the line y = x to find the dividing line, which translates back to y = (b/a)x for the original ellipse. This approach effectively determines the angle of inclination for equal area division.
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Hi all,
How should I divide quarter of an ellipse into two equal havles? At what angle should I divide so that the 2 parts are equal?
Any hint is a privilege.
Thanks in advance
Regards
Suji
 
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I would begin with an ellipse in standard form centered at the origin:

$$\left(\frac{x}{a} \right)^2+\left(\frac{y}{b} \right)^2=1$$

Converting to polar coordinates, we may write:

$$r^2\left(\frac{\cos^2(\theta)}{a^2}+ \frac{\sin^2(\theta)}{b^2} \right)=1$$

Solving for $r^2$, we obtain:

$$r^2=\frac{(ab)^2}{a^2\sin^2(\theta)+b^2 \cos^2(\theta)}$$

Next, using the formula for area in polar coordinates, we obtain:

$$\int_0^{\beta}\frac{1}{a^2\sin^2(\theta)+b^2\cos^2(\theta)}\,d\theta= \int_{\beta}^{\frac{\pi}{2}}\frac{1}{a^2\sin^2( \theta)+b^2\cos^2( \theta)}\,d\theta$$

Applying the FTOC, we then obtain:

$$\left[\tan^{-1}\left(\frac{a}{b}\tan(\theta) \right) \right]_0^{\beta}=\left[\tan^{-1}\left(\frac{a}{b}\tan(\theta) \right) \right]_{\beta}^{\frac{\pi}{2}}$$

This gives us:

$$\tan^{-1}\left(\frac{a}{b}\tan(\beta) \right)=\frac{\pi}{4}$$

Taking the tangent of both sides:

$$\frac{a}{b}\tan(\beta)=1$$

Solve for $\beta$:

$$\beta=\tan^{-1}\left(\frac{b}{a} \right)$$

A much simpler technique would be to begin with the ellipse:

$$\left(\frac{x}{a} \right)^2+\left(\frac{y}{b} \right)^2=1$$

Now stretch the vertical axis by a factor of $$\frac{a}{b}$$ such that the ellipse now becomes a circle of radius $a$. We know the line $y=x$ will divide the first quadrant area of the circle into two equal halves.

Now shrink the vertical axis back to where it started, by a factor $$\frac{b}{a}$$, and the dividing line is now:

$$y=\frac{b}{a}x$$

And we see the angle of inclination of this line is:

$$\beta=\tan^{-1}\left(\frac{b}{a} \right)$$
 
Thanks a million sir MarkFL.
This helps me.:D
 
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