1). AD = DC, значит, треугольник ADC — равнобедренный и <img src="http://tex.z-dn.net/?f=%5Cangle+DAC+%3D+%5Cangle+DCA+%3D+20%5E%7B%5Ccirc%7D» id=»TexFormula1″ onerror=»texError(this)» title=»angle DAC = angle DCA = 20^{circ}″ alt=»angle DAC = angle DCA = 20^{circ}″ align=»absmiddle» class=»latex-formula»>.
<img src="http://tex.z-dn.net/?f=%5Cangle+ADC+%3D+180%5E%7B%5Ccirc%7D+-+%28%5Cangle+DAC+%2B+%5Cangle+DCA%29+%3D+180%5E%7B%5Ccirc%7D+-+2%2A20%5E%7B%5Ccirc%7D+%3D+140%5E%7B%5Ccirc%7D» id=»TexFormula2″ onerror=»texError(this)» title=»angle ADC = 180^{circ} — (angle DAC + angle DCA) = 180^{circ} — 2*20^{circ} = 140^{circ}″ alt=»angle ADC = 180^{circ} — (angle DAC + angle DCA) = 180^{circ} — 2*20^{circ} = 140^{circ}″ align=»absmiddle» class=»latex-formula»>.
2). AD — биссектриса, значит, <img src="http://tex.z-dn.net/?f=%5Cangle+BAD+%3D+%5Cangle+DAC+%3D+20%5E%7B%5Ccirc%7D» id=»TexFormula3″ onerror=»texError(this)» title=»angle BAD = angle DAC = 20^{circ}″ alt=»angle BAD = angle DAC = 20^{circ}″ align=»absmiddle» class=»latex-formula»> и
<img src="http://tex.z-dn.net/?f=%5Cangle+A+%3D+2%2A+%5Cangle+DAC+%3D+2%2A20%5E%7B%5Ccirc%7D+%3D+40%5E%7B%5Ccirc%7D» id=»TexFormula4″ onerror=»texError(this)» title=»angle A = 2* angle DAC = 2*20^{circ} = 40^{circ}″ alt=»angle A = 2* angle DAC = 2*20^{circ} = 40^{circ}″ align=»absmiddle» class=»latex-formula»>.
<img src="http://tex.z-dn.net/?f=%5Cangle+B+%3D+180%5E%7B%5Ccirc%7D+-+%28%5Cangle+C+%2B+%5Cangle+A%29+%3D+180%5E%7B%5Ccirc%7D+-+%2820%5E%7B%5Ccirc%7D+%2B+40%5E%7B%5Ccirc%7D%29%3D+120%5E%7B%5Ccirc%7D» id=»TexFormula5″ onerror=»texError(this)» title=»angle B = 180^{circ} — (angle C + angle A) = 180^{circ} — (20^{circ} + 40^{circ})= 120^{circ}″ alt=»angle B = 180^{circ} — (angle C + angle A) = 180^{circ} — (20^{circ} + 40^{circ})= 120^{circ}″ align=»absmiddle» class=»latex-formula»>.