Question

已知X+Y+Z=a,XY+YZ+XZ=b,求X*X+Y*Y+Z*Z的值

Answer 1

（x+y+z）(x+y+z）=a*a，展开x*x+y*y+z*z+2(x*y+x*z+y*z)=a*a，代入得到

x*x+y*y+z*z+2b=a*a，即x*x+y*y+z*z=a*a-2b