The largest and the shortest distance of the earth from the sun are r1 and r2, its distance from the sun when it is at the perpendicular to the major axis of the orbit drawn from the sun
The earth moves around the sun is elliptical path. So by using the properties of ellipse
r1=(1+e)a and r2=(1−e)a
⇒a=r1+r22 and r1r2=(1−e2)a2
where a=semi major axis
b=semi minor axis
e=eccentricity
Now required distance=semi latusrectum=b2a
=a2(1−e2)a=(r1r2)(r1+r2)2=2r1r2r1+r2