设a+b/10=b+c/11=c+a/15=t t>0
a+b=10t
b+c=11t
c+a=15t 相加
2a+2b+2c=36t
a+b+c=18t
c=8t a=7t b=3t
cosC=(a^2+b^2-c^2)/2ab=(49t^2+9t^2-64t^2)/2*3t*7t=-1/7
设a+b/10=b+c/11=c+a/15=t t>0
a+b=10t
b+c=11t
c+a=15t 相加
2a+2b+2c=36t
a+b+c=18t
c=8t a=7t b=3t
cosC=(a^2+b^2-c^2)/2ab=(49t^2+9t^2-64t^2)/2*3t*7t=-1/7