Answer: Triangle ABC is a right triangle, because it satisfies the Pythagorean Theorem, which states that in a right triangle, the square of the length of the hypotenuse (the side opposite the right angle) is equal to the sum of the squares of the lengths of the other two sides.
c^2 = a^2 + b^2
25^2 = 17^2 + 21^2
625 = 289 + 441
625 = 730
Since it is a right triangle, angle A is the angle opposite side a, which is the side that is the length of 17.
A right triangle also has one angle which is always 90 degree, that is the angle opposite the hypotenuse, so the remaining angle is A, which is 90- angle C, which is
90 - 42.3 = 47.7 degree.
Therefore the measure of angle A is 47.7°
So the correct answer is C. 42.3°
Explanation: