이등변삼각형의 각 구하기

문제

다음 그림과 같이 $\overline{AB} = \overline{AC}$인 이등변삼각형 $ABC$가 있습니다. $\,\angle ABC = 50^\circ$일 때, $\angle BAC$의 크기는 몇 도일까요?

<!-- Equality marks for sides AB and AC -->
<!-- Midpoint of AB: (150, 140.4125). Angle of AB: atan2(80.825-200, 200-100) = -50 degrees.
     Dashes should be perpendicular, so rotate by -$50 + 90 = 40$ degrees. -->
<g transform="translate(150, 140.4125) rotate(40)">
  <line x1="-1.5" y1="-4" x2="-1.5" y2="4" stroke="#1F2937" stroke-width="2"/>
  <line x1="1.5" y1="-4" x2="1.5" y2="4" stroke="#1F2937" stroke-width="2"/>
</g>
<!-- Midpoint of AC: (250, 140.4125). Angle of AC: atan2(80.825-200, 200-300) = -130 degrees.
     Dashes should be perpendicular, so rotate by -$130 + 90$ = -40 degrees. -->
<g transform="translate(250, 140.4125) rotate(-40)">
  <line x1="-1.5" y1="-4" x2="-1.5" y2="4" stroke="#1F2937" stroke-width="2"/>
  <line x1="1.5" y1="-4" x2="1.5" y2="4" stroke="#1F2937" stroke-width="2"/>
</g>

<!-- Angle arc and label for angle ABC (50 degrees) -->
<!-- Center at B(100, 200). Radius 40. -->
<!-- Start point: (100 + 40*cos(0), 200 + 40*sin(0)) = (140, 200) -->
<!-- End point: (100 + 40*cos(-50), 200 + 40*sin(-50)) = (125.708, 169.36) -->
<!-- Path command draws a clockwise arc from 0 to -50 degrees relative to B's x-axis. -->
<path d="M 140 200 A 40 40 0 0 0 125.708 169.36" class="line"/>
<text x="155" y="185" class="angle-label">50°</text>

<!-- Points as small black circles -->
<circle cx="200" cy="80.825" r="3" class="point"/>
<circle cx="100" cy="200" r="3" class="point"/>
<circle cx="300" cy="200" r="3" class="point"/>

<!-- Point Labels -->
<text x="200" y="70" class="point-label">A</text>
<text x="90" y="215" class="point-label">B</text>
<text x="310" y="215" class="point-label">C</text>