余弦定理公式还是：c^2=a^2+b^2-2*a*b*COS(C)
b^2=a^2+c^2-2*a*c*COS(B)
a^2=b^2+c^2-2*b*c*COS(A)
角小于90度时,COS(角)的值为正；角大于90度时,COS(角)的值为负.