Answer
α = $60^{\circ}$
Work Step by Step
$\angle A = 30^{\circ}$ (given)
$\angle ADC = 90^{\circ}$
$\angle α = 180^{\circ}$ - (90$^{\circ}$ + 30$^{\circ}$) (angle sum of triangle is 180$^{\circ}$)
= 60$^{\circ}$
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