如果是30/60/90，那么边长比就是 a : b : c = 1 : √3 : 2c = 0.55 * 2 / √3 = a = 0.55 / √3 =
c=b/cos30°=0.55/(√3/2)≈0.6351a=btan30°=0.55×√3/3≈0.31735
cos30°=b/c=√3/2 tan30°=a/b=√3/3
a/sinA=b/sinB=c/sinC
