Property of elliptical orbits I've studied that the major axis of a planets orbit = ( radius of perihelion + radius of aphelion) . But then  how can we calculate the value of the minor axis if we are provided with the values of radius of perihelion and radius of aphelion only ?
If $r_p$ is the perihelion distance (the closest distance of the planet in its elliptical orbit from the Sun), and if $r_a$ is the aphelion distance (the farthest distance from the Sun), the major axis is
$$
2a = r_a+r_p
$$
where $a$ is the semi-major axis (half of the major axis).
The eccentricity of the orbit can be calculated from the perihelion and aphelion distances using
$$
e = \frac{r_a-r_p}{r_a+r_p}
$$
Once the eccentricity is known, the semi-minor axis can be calculated from
$$
b = a\sqrt{1-e^2}
$$
The minor axis, of course, is twice the semi-minor axis, or $2b$.