Each equation represents a proportional relationship. Choose the equations for which the constant of proportionality is 1/ 4

Find the value of the cosine for angle a

Can someone me out here number 4 plz

If the ratio of sum of the first m and n terms of an ap is m2 : n2 , show that the ratio of its mth and nth terms is (2m − 1) : (2n − 1).