Question 729053
using the cosine rule is a waste of time!
if you draw the triangle, label it and examine the side lengths. They all can be divided by 4 giving a 3, 4, 5 triangle and is therefore a right angled triangle. so the angle CAB is 90 degrees.
HOWEVER if the cosine rule MUST be used........
the cosine rule is
{{{a^2=b^2+c^2-2bcCos(A)}}}
rearranging gives
{{{Cos(A)=(b^2+c^2-a^2)/(2bc)}}}
substituting values gives us
{{{Cos(A)=(4^2+3^2-5^2)/(2*4*3)}}}
evaluating gives
{{{cos(A)=(0)/24}}}
i.e. Cos(A)=0
therefore A = 90 degrees