Malia tried to prove that cos(0) =sin(90°-0) using the following diagram. her proof is not correct

  the  net force  is 180n to the  right.  force  is a vector so it shows magnitude and direction. -basically  we applied 200n force  to the  right, but  we  had  20n  of resistance to overcome (the  friction force). friction always points opposite of where  we're pushing, thus it would be subtracted from the overall  force.