f(x1)-f(x2)=t(x1^2-x2^2)+4t(x1-x2)=t(x1+x2)(x1-x2)+4t(x1-x2)=(x1-x2)(t(x1+x2)+4t)=(x1-x2)(t(1-t)+4t)=(x1-x2)(5-t)t
t>5,5-t0
f(x1)
f(x1)-f(x2)=t(x1^2-x2^2)+4t(x1-x2)=t(x1+x2)(x1-x2)+4t(x1-x2)=(x1-x2)(t(x1+x2)+4t)=(x1-x2)(t(1-t)+4t)=(x1-x2)(5-t)t
t>5,5-t0
f(x1)