Question 1194105
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This problem is about a triangle  STR  inscribed into a circle.


This triangle has side  SR  as a diameter of the circle - - - hence,  the triangle is a right-angled triangle
with the right angle  T  opposite to side  SR.


Also,  the triangle has leg  ST  one half of the hypotenuse  SR - - - hence,  it is  30-60-90  degrees triangle.



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