cosA=3/4, 4/5, 5/6 ....as long as cosA * cosA >1/2

sin3A/sinA=3-4* (sinA)^2
sin(pi-4A)/sinA=sin(4A)=4cosA*(1-2*sinA*sinA)
they should be positive,so sinA * sinA <1/2
so that cosA * cosA > 1/2

you can pick cosA=3/4, 4/5, 5/6 ..........