At the first meeting of the Tennis Club, the 7 members decided to have a tournament in which every player will play a match against every other player. How many matches will there be in the tournament?

21 matches

Step-by-step explanation:

This is a combination problem where we have to select two member out of 7 members

So we want calculation of  7C2

   7C2 = 7!/[(2!)(7-2)!]

   (7*6*5*4*3*2*1)/[(2*1) *(5*4*3*2*1]

   (7*6)/2    ~  Note everything that easily cancels here!

   42/2

   21 matches

reduce the friction between its moving parts

step-by-step explanation:

in fact, by reducing the friction between the moving parts of the machine, it is possible to reduce the energy wasted due to this friction; therefore, more input energy is converted into useful work, and this will improve the efficiency of the machine.