In (bigtriangleup) XYZ, XY = 13cm, YZ = 9cm, XZ = 11cm and XYZ = (theta). Find cos(theta)

^{o}-
**A.**

(frac{4}{39}) -
**B.**

(frac{43}{39}) -
**C.**

(frac{209}{3}) -
**D.**

(frac{43}{78})

##### Correct Answer: Option D

##### Explanation

cos(theta) = (frac{13^2 + 9^2 – 11^2}{2(13)(9)})

= (frac{169 + 81 – 21}{26 times 9})

cos(theta) = (frac{129}{26 times 9})

= (frac{43}{78})