f(x1)-f(x2)
=ax1^2+2ax1+4-(ax2^2+2ax2+4)
=a(x1²-x2²)+a(x1-x2)
=a(x1+x2)(x1-x2)+a(x1-x2)
=a(x1-x2)(x1+x2+1)
∵X1+X2=1-a,a>3
∴1-a
f(x1)-f(x2)
=ax1^2+2ax1+4-(ax2^2+2ax2+4)
=a(x1²-x2²)+a(x1-x2)
=a(x1+x2)(x1-x2)+a(x1-x2)
=a(x1-x2)(x1+x2+1)
∵X1+X2=1-a,a>3
∴1-a