c² = (a-x)²+ h² = a² 2ax + x2 + h² = a² - 2ax + b² =a2-2a(bcosC) + b² = a² + b²-2ab cos C Apply Pythagorean Theorem to AABD Next 2 steps replace x w/ a, b, c, C: For AADC, b²x² + h² COSC x == ⇒x=bcosC b ​