The straight line on the plane (which can't be expressed by the equation of the equal sign of the space straight line) can be expressed by a binary linear equation, which is equivalent to saying that the binary quadratic equation can be decomposed into the product of two linear equations with real coefficients (which may be binary or unitary), and the original equation is equal to zero, which is equivalent to one linear equation or the other linear equation is zero. For example, x 2-y 2 = 0 is equivalent to (x+y)(x-y)=0, that is, x+y=0 and x-y=0.