a^2-16b^2-c^2+6ab+10bc
=a^2-16b^2-c^2+8ab-2ab+8bc+2bc+ac-ac
=(a^2+ac-2ab)+(8ab+8bc-16b^2)+(-ac-c^2+2bc)
=a(a+c-2b)+8b(a+c-2b)-c(a+c-2b)
=(a+c-2b)(a+8b-c)
由于abc是三角形三边
a+8b-c>a+b-c>0
所以a+c-2b=0
a+c=2b