Answer:
cos ∠CBD = - 4√41 / 41
Step-by-step explanation:
AB = 4 ΔABC = (4 x AC) / 2 = 10
AC = 5
BC = √5² + 4² = √41
cos ∠CBD = cos (180° - ∠CBA) = cos 180° cos ∠CBA + sin 180° sin ∠CBA
(cos 180° = - 1 sin 180° = 0 cos ∠CBA = 4 / √41 )
cos ∠CBD = (-1) x cos ∠CBA = - 4 / √41 = - 4√41 / 41
