49 * sin^2a = 49* 25/49 = 25

cosa = sqrt(1 - sin^2a) = sqrt(1 - 25/49) = sqrt(24/49) = (2sqrt(6))/ 7

49(1 - ((2sqrt(6))/ 7)^2) = 49(1 - (24/49)) = 49(25/49) = 25