In any triangle, the smallest angle is opposite to the shortest side.
In the given triangle, the smallest angle is opposite to the 9 cm side and is
concluded between the 11 cm and 13 cm sides.
Write the cosine law for the shortest side of 9 cm
9^2 = 11^2 + 13^2 - 2*11*13*cos(a)
where "a" is the angle under the question.
From this equation,
cos(a) = = = = 0.730769231.
So, the angle "a" is
a = arccos(0.730769231) = 43.04907978 degrees. ANSWER
You may round it as you want.
