任取x1>x2在(-无穷,1】
所以f(x1)-f(x2)
=2x1^2-4x1+3-2x2^2+4x2-3
=2(x1^2-x2^2)-4(x1-x2)
=2(x1-x2)(x1+x2-2)
因为x1>x2在(-无穷,1】
所以
f(x1)-f(x2)