Problem 25. Finding the Area (II)

In the figure below, a circle with a radius of 2 is tangent to the center of the side of a half-circle with a radius of 3, and a square is surrounded by their intersection.
What is the area of the green part?
The answer to the question is \(1.349\).
In the first step, we find the length of the sides of the square, which is approximately 2.36822, and the x coordinate of the left side of the square which is \(x_4 \simeq -1.18411\). In the second step, we find the area of the intersection of the red circle and the green semicircle. We know that \(y_3 = 2.25\). Now, putting it in the semicircle formula, we get: \[x_3^2+y_3^2 = 3^2 \quad \implies \quad x_3 \simeq -1.98431\] Now, if we look closely at the target area, it can be divided into 3 parts:
green section which is located between the green semicircle and the line \(x=2\).
red section which is located between the line \(x=2\) and the lower half of the red circle.
blue section which is located between the upper half of the red circle and the line \(x=2\).
Now we get the area of each part by integral: \[\begin{aligned} A(\textsf{blue}) &= \int_{-2}^{x_3}\sqrt{4-x^2}dx \simeq 0.00262 \\ A(\textsf{green}) &= \int_{x_3}^{x_4}\sqrt{9-x^2}-2 dx \simeq 0.42644 \\ A(\textsf{red}) &= \int_{-2}^{x_4}2 - (-\sqrt{4-x^2} + 2) dx \simeq 0.92011 \end{aligned}\] By adding the above three values, the answer to the problem is approximately 1.349.