Prove that the area of the trangles are equal.

Let

• $*sbh$ stands for triangles have same base and height.
• $*c$ stands for triangles are congruent.

We have

1. $A_1 \stackrel{*sbh}{=} B_1 \stackrel{*c}{=} B_3 \stackrel{*c}{=} A_3$,

2. $A_3 \stackrel{*c}{=} B_3 \stackrel{*c}{=} B_4 \stackrel{*sbh}= A_4$,

3. $A_2 \stackrel{*c}= A_3$

It is useful to know that the area of a triangle can be calculated by $$\frac12ab\sin\theta$$ where $a$ and $b$ are two side lengths of the triangle and $\theta$ is the angle between those two side lengths.

Let $\alpha$ be the angle in $A_3$ formed by the side lengths $b$ and $c$. Then, the angle formed by the side lengths $b$ and $c$ in $A_4$ is $\pi-\alpha$. Since $\sin\alpha=\sin(\pi-\alpha)$, $A_3$ and $A_4$ have the same area.

This can be applied to the other triangles in your diagram.