AD=AC maka nilai x adalah??

AD = AC
ΔADC = Δ Sama Kaki = 2 sudut sama besar
∠CAD = 40°
∠BDA dan ∠CDA = Berpelurus
∠Δ = 180°

Maka
40 + a + a = 180
2a = 180 - 40
2a = 140
a = 140 ÷ 2
a = 70
∠CDA = 70°

70 + 4x -10 = 180
4x = 180 - 60
4x = 120
x = 120 ÷ 4
x = 30° (C)