OG详解-OG3 数学2 Q9

Choice D is correct. The sum of the angle measures of a triangle is 180º. Adding the measures of angles B and C gives 52 +17 = 69º. Therefore, the measure of angle A is 180-69 =111º.
Choice A is incorrect and may result from subtracting the sum of the measures of angles B and C from 90º, instead of from 180º. Choice B is incorrect and may result from subtracting the measure of angle C from the measure of angle B . Choice C is incorrect and may result from adding the measures of angles B and C but not subtracting the result from 180º.