Cho: bz + ay/x*( - ax + by + cz) = cx + az/y*(ax - by + cz) = ay + bx/z(ax + by - cz). Chứng minh x/a(b^2 + c^2 - a^2) = y/b(a^2 + c^2 - b^2) = z/c( a^2 + b^2 - c^2)