cosine formula

Let b = k , c = 3k since b : c = 1 : 3.By cosine formula :a² = b² + c² - 2bc cosA
25² = k² + (3k)² - 2(k)(3k) cos35°
625 = 10k² - 6cos35° k²
625 = k² (10 - 6cos35°)
k² = 625 / (10 - 6cos35°)The area of ΔABC = (1/2) bc sinA
= (1/2) k(3k) sin35°
= (3/2) sin35° k²
= (3/2) sin35° * 625 / (10 - 6cos35°)
= 105.746... cm²