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 ellipser1=(1+e)a and r2=(1−e)a⇒a=r1+r22 and r1r2=1−e2a2where a = semi major axisb = semi minor axise = eccentricityNow required distance = semi latusrectum =b2a=a2(1−e2)a=(r1r2)(r1+r2)/2=2r1r2r1+r2