a*ac-a*ab+b*ba-b*bc=0
a²c-a²b+b²a-b²c=0
a²c-b²c -a²b+b²a =0
c(a²-b²) –ab(a-b) =0
c(a-b)(a+b) –ab(a-b) =0
(a-b)[c (a+b) –ab] =0
(a-b)(ac +bc –ab) =0
a=b
等腰三角形.