(sinA)^2+(sinB)^2=5(sinC)^2
由正弦定理得：
a^2+b^2=5c^2≥2ab
ab≤5c^2/2
由余弦定理：
cosC=(a^2+b^2-c^2)/2ab
= 4c^2/2ab ≥4c^2/5c^2=4/5
(sinC)^2=1-(cosC)^2≤9/25
sinC≤3/5