sin^2 a + cos^2 a = 1
cos a=21/29
cos^ 2 a =441/841
sin^ 2 a = 1 - cos^ 2 a
sin^ 2 a = 1 - 441/841
sin^ 2 a = 400/841
sin a=√400/√841=20/29
sin a = 20/29
