△=[2(B-A)]^2-4(C-B)(A-B)
=4A^2+4B^2-8AB-4B^2+4BC-4AC+4AB
=4A^2-4AB+4BC-4AC=0
A^2-AB+BC-AC=0
A(A-C)-B(A-C)=0
A(A-C)=B(A-C)
A=B
△ABC形状为等腰△
△=[2(B-A)]^2-4(C-B)(A-B)
=4A^2+4B^2-8AB-4B^2+4BC-4AC+4AB
=4A^2-4AB+4BC-4AC=0
A^2-AB+BC-AC=0
A(A-C)-B(A-C)=0
A(A-C)=B(A-C)
A=B
△ABC形状为等腰△