c/b=sinC/sinB=sin3B/sinB=(3sinB－4sin^3B)/sinB=3-4sin^2B因为C=3B，所以B<45度所以 sinB<√2/2 所以sin^2B<1/2所以c/b=3-4sin^2B>3-4*(1/2)=3-2=1即 c/b>1
