Ex9-1

||f\ast g||_p=\big(\int|f\ast g(x)|^{p}dx\big)^{1/p}
=(\int\int|f(x-t)g(t)dt|^p dx)^{1/p}
\leq\int(\int|f(x-t)g(t)|^p dx)^{1/p}dt
 by Minkowski's Integral Inequality (exercise 8-8 of the textbook) 
=\int(\int|f(x-t)|^p dx)^{1/p}\ |g(t)|dt
=||f||_p||g||_1