Concept :-
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We have studied the law which states that the sum of all angles in a triangle is 180°, this is known as angle sum property of a triangle. We can solve this question based on that property.
Solution :-
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Let us assume the missing angle to be x,
Therefore,
→ 66° + 71° + x = 180° ( Angle sum property of a triangle )
→ 137° + x = 180°
→ x = 180° - 137°
→ x = 43°
Thus, the measure of the missing angle is 43°.
Hope that helps :)