\ end {array}\] Ya quef′(c1)≤f′(c2), tenemos\[t f\left(x_{t}\right)-t f\left(x_{1}\right)=f^{\prime}\left(c_{1}\right) t(1-t)\left(x_{2}-x_{1}\righ...\ end {array}\] Ya quef′(c1)≤f′(c2), tenemostf(xt)−tf(x1)=f′(c1)t(1−t)(x2−x1)≤f′(c2)t(1−t)(x2−x1)=(1−t)f(x2)−(1−t)f(xt). reordenando términos, obtenemosf(xt)≤tf(x1)+(1−t)f(x2). Por lo tanto,f es convexo.