Cosine law formula is:
[tex] a^{2} = b^{2} + c^{2} - 2*b*c*cos(A)[/tex]
where A is an angle opposite from side a.
If angle a is 90 degrees that means that cos(90) is 0 which leaves us with
[tex] a^{2} = b^{2} + c^{2} [/tex]
This is Pythagoras theorem where a is the hypothesis and b and c are smaller sides of triangle
Answer is A)
