Mathematics

Mathematics

Problem

In a quadrilateral ABCDABCD , ACAC is the bisector of the ∠(AB,AD)\angle (AB, AD) which is 2π3\frac{2\pi}{3} , 15∣AC∣=3∣AB∣=5∣AD∣15|\mathbf{AC}| = 3|\mathbf{AB}| = 5|\mathbf{AD}| , then cos⁡(BA,CD)\cos(\mathbf{BA}, \mathbf{CD}) is equal to a. −1472-\frac{\sqrt{14}}{7\sqrt{2}} b. −2173-\frac{\sqrt{21}}{7\sqrt{3}} c. 27\frac{2}{\sqrt{7}} d. 2714\frac{2\sqrt{7}}{14}

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