Equation: x? +y？ +d 1x+e 1y+f 1+λ(x？ +y？ +D2x+E2y+F2)=0 …… ①，
When λ≦- 1, equation ① represents the equation that the circle system passes through the intersection of circle A and circle B. When λ=0, it represents circle A, but it cannot represent circle B.: When λ=- 1, if circle A intersects circle B, equation ① represents the linear equation where the common chord of circle A and circle B is located; When circle A is tangent to circle B, equation ① represents the common tangent equation of circle A and circle B; When two circles are separated, Equation ① represents an equation perpendicular to the straight line connecting the two circles. This conclusion is often used to solve problems related to circles, which can get twice the result with half the effort.