According to the known conditions, the angle ACB = 2° angle DBC is obtained, and the cos angle ACB is calculated from the triangle ABC, and then the cos angle ACB is calculated, and then the cos angle DBC is calculated. Finally, according to the formula of cos angle DBC = (BD 2+BC 2-CD 2)/(2bd * BC), the equation is solved under BD= radical sign to get 42. Think independently, be familiar with triangles, and you will always drop O(∩_∩)O haha ~