3x^2+4xy-y^2=4x^2+4xy-y^2-x^2=(2x-y)^2-x^2=(x-y)(3x-y)a^2+b^2+c^2=ab+bc+ca2a^2+2b^2+2c^2=2ab+2bc+2ca2a^2+2b^2+2c^2-2ab-2bc-2ca=0(a-b)^2+(a-c)^2+(b-c)^2=0所以三角形是等边三角形