∵AB=AC∴∠ABC=∠Cl∵∠MAC=∠ABC+∠C∴∠MAC=2∠C∵AD平分∠MAC∴∠MAC=2∠DAC∴∠DAC=∠C∴AD//BC∴∠D=∠DBC∵BD平分∠ABC∴∠ABD=∠DBC∴∠ABD=∠D∴三角形ABD是等腰三角形
