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ABC is a right angle triangle.

we know tha sum of three angles of a triangle is 180 deg.

angle C = 90 - angle A
In triangle ABD,  angle ADB = 90 deg.
     Hence, angle ABD = 90 - angle A = angle C
In triangle CBD, angle 90 - angle C = angle B ,,  as angle BDC = 90 deg.

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