a^2+b^2+c^2+50=6a+8b+10c, a^2+b^2+c^2-6a-8b-10c+50=0, a^2-6a+9+b^2-8b+16+c^2-10c+25=0, (a-3)^2+(b-4)^2+(c-5)^2=0, a=3,b=4,c=5 勾股定理逆定理，判断是直角三角形
