1.△ABD∽△ACE!
∠1=∠2
∠BAC=∠DAE
∠3=∠4
△BAC∽△DAE
AB/AD=AC/AE
AB/AC=AD/AE
∠1=∠2
△ABD∽△ACE.
2.2x^2-7xy+6y^2=(2x-3y)(x-2y)=0
2x-3y=0,或x-2y=0
x/y=3/2,或x/y=2.
3.是AB=BC=a!
由对称性知,MN‖AC
MN/AC=BM/BC
由角平分线定理：
BM/CM=AB/AC=a/b
BM/BC=BM/(BM+CM)=a/(a+b)
MN/AC=a/(a+b)
MN=ab/(a+b)
选择：C.