Step-by-step explanation:
first we use Pythagoras to get t :
c² = a² + b²
with c being the Hypotenuse.
so,
t² = r² + s² = 10² + 31² = 100 + 961 = 1061
t = sqrt(1061)
and then we use the law of sine :
a/sin(A) = b/sin(B) = c/sin(C)
with the angles being opposite of their related sides.
so, r/sin(R) = t/sin(T) = t/sin(90) = t/1 = t
sin(R) = r/t = 10/sqrt(1061) = 0.30700278...
R = 17.8786966... ≈ 17.9°
