7. A resistor with resistance $R$ is traversed by a current varying in time with intensity $I ( t ) = I _ { 0 } \frac { a } { t }$, with $t > 0$ and the positive constants $I _ { 0 }$ and $a$ expressed, respectively, in amperes and in seconds. Knowing that the power dissipated in the resistor due to the Joule effect is $P ( t ) = R I ^ { 2 } ( t )$, determine its average value on the interval $[ 2 a ; 3 a ]$.