Answer :
4^2 = 3^2 + 2^2 - 2 * 3 * 2 * cosC <=> 16 = 13 - 12 * cosC <=> 12 * cosC = -3 <=> cosC = - 1/4.
The applicable formula is known as the law of cosines
C² = a² + b² -2abcosC
cosC = ( a² + b² - c²)/2ab
= ( 2² + 3² - 4²)/2*2*3
= (13-16)/12
= -3/12
= -1/4
C² = a² + b² -2abcosC
cosC = ( a² + b² - c²)/2ab
= ( 2² + 3² - 4²)/2*2*3
= (13-16)/12
= -3/12
= -1/4
