Ta có:
∠ADC = $180^{0}$ - ∠ADB
= $180^{0}$ - $80^{0}$ = $100^{0}$
Có:
∠BAD = $180^{0}$ - $80^{0}$ - ∠B
∠CAD = $180^{0}$ - $100^{0}$ - ∠C
Mà: ∠BAD = ∠CAD (AD là phân giác)
⇒ $180^{0}$ - $80^{0}$ - ∠B = $180^{0}$ - $100^{0}$ - ∠C
⇔ - $80^{0}$ - ∠B = - $100^{0}$ - ∠C
Mà: ∠B = 1,5∠C
⇒ - $80^{0}$ - 1,5∠C = - $100^{0}$ - ∠C
⇔ 0,5∠C = $20^{0}$
⇔ ∠C = $40^{0}$
⇒ ∠B = 1,5∠C = 1,5 . $40^{0}$ = $60^{0}$
⇒ ∠A = $180^{0}$ - $60^{0}$ - $40^{0}$ = $80^{0}$
