Problem 1666

1. Given $\Delta ABC$ with $\angle CAB = 75^\circ$ and $\angle ACB = 45^\circ$. We need to find $\angle BMC$ where $BC$ is extended to $T$ such that $BC = CT$ and $M$ is the midpoint of $AT$.
2. Draw the altitude from vertex $A$ to $BC$ and call the intersection point $H$. This gives us two right triangles $\Delta ABH$ and $\Delta AHC$.
3. Since $\angle CAB = 75^\circ$ and $\angle ACB = 45^\circ$, we can find $\angle BAH$ and $\angle HAC$:
$$\angle BAH = 75^\circ - 45^\circ = 30^\circ$$
$$\angle HAC = 45^\circ$$
4. Let $BH = x$. Then, using trigonometric ratios in $\Delta ABH$ and $\Delta AHC$:
$$AB = 2x \quad (\text{since } \sin 30^\circ = \frac{1}{2})$$
$$AH = x\sqrt{3} \quad (\text{since } \tan 30^\circ = \frac{1}{\sqrt{3}})$$
$$HC = x\sqrt{3} \quad (\text{since } \tan 45^\circ = 1)$$
$$AC = x\sqrt{6} \quad (\text{using Pythagoras' theorem in } \Delta AHC)$$
5. Since $BC = CT$, $AM = MT$ implies $M$ is the midpoint of $AT$. Therefore, $MC$ is the midsegment of $\triangle ABT$ and $MC = x$.
6. $BM$ and $AC$ are medians. Let $K$ be the intersection point of $BM$ and $AC$. $K$ is the centroid of $\triangle ABC$.
7. The centroid divides each median in the ratio $2:1$. Therefore:
$$\frac{AK}{KC} = 2 \implies AK = \frac{2x\sqrt{2}}{\sqrt{3}}, \quad KC = \frac{x\sqrt{2}}{\sqrt{3}}$$
8. Using the Law of Cosines in $\triangle KMC$:
$$KM^2 = KC^2 + CM^2 - 2 \cdot KC \cdot CM \cdot \cos 75^\circ$$
We know $\cos 75^\circ = \frac{\sqrt{6} - \sqrt{2}}{4}$, so:
$$KM^2 = \left(\frac{x\sqrt{2}}{\sqrt{3}}\right)^2 + x^2 - 2 \cdot \frac{x\sqrt{2}}{\sqrt{3}} \cdot x \cdot \frac{\sqrt{6} - \sqrt{2}}{4}$$
Simplifying the expression:
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{12} - \sqrt{6})}{2\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(2\sqrt{3} - \sqrt{6})}{2\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
$$KM^2 = \frac{2x^2}{3} + x^2 - \frac{x^2(\sqrt{3} - \sqrt{2})}{\sqrt{3}}$$
\[
KM^2 =