The perihelion is the point in a planet's orbit where it is closest to the sun.
The equation for the orbit of a planet around the sun depends on various factors, such as the shape of the orbit, the mass of the planet, and the gravitational force between the planet and the sun. Kepler's laws of planetary motion provide a framework for understanding the motion of planets in orbit, and the equations used to describe these orbits are generally based on these laws. The specific equation for the orbit of planet A would depend on the specific parameters of that planet's orbit.