令a/sinA = b/sinB = c/sinC = k
a : b : c=ksinA : ksinB : ksinC=2:3:4
设a=2x,b=3x,c=4x
cosC=(a²+b²-c²)/2ab=(4x²+9x²-16x²)/(2*2x*3x)=-1/4
令a/sinA = b/sinB = c/sinC = k
a : b : c=ksinA : ksinB : ksinC=2:3:4
设a=2x,b=3x,c=4x
cosC=(a²+b²-c²)/2ab=(4x²+9x²-16x²)/(2*2x*3x)=-1/4