Geometry Practice Test 2

AB = BC
∠BAC = 40°
∠BAC = ∠BCA = 40°
In ΔABC,
∠BAC + ∠BCA + ∠ABC = 180°
40° + 40° + ∠ABC = 180°
∠ABC = 180° - 80°
∠ABC = 100°
∠ABC + ∠ADC = 180° (sum of two opposite angles is 180° in a cyclic quadrilateral.)
100° + ∠ABC = 180°
∠ADC = 180° - 100°
∠ADC = 80°