Given,
l1,l2,.......,l100 are in A.P with common difference d1.
So from property of A.P we can say,
l2=l1+d1
l3=l1+2d1
⋮
l100=l1+99d1
Also given,
w1,w2,......,w100 are in A.P with common difference d2.
∴ From the property of A.P we can say,
w2=w1+d2
w3=w1+2d2
⋮
w100=w1+99d2
Now, also given,
d1d2=10
and Ri is a rectangle whose length is li and width is wi and area Ai.
∴ We know, area of rectangle
Ai=li×wi
∴ A51=l51×w51
and A50=l50×w50
Given, A51−A50=1000
⇒(l1+50d1)(w1+50d2)−(l1+49d1)(w1+49d2)=1000
⇒[l1w1+2500d1d2+50(l1d2+d1w1)]−[l1w1+49×49d1d2+49(l1d2+w1d1)]=1000
⇒[(50)2d1d2−(49)2d1d2]+(50−49)(l1d2+d1w1)=1000
⇒(99×1)d1d2+l1d2+d1w1=1000
⇒99×10+l1d2+w1d1=1000
⇒l1d2+w1d1=10
Now,
A100−A90
=l100.w100−l90.w90
=(l1+99d1)(w1+99d2)−(l1+89d1)(w1+89d2)
=[l1w1+(99)2d1d2+99(l1d2+w1d1)]−[l1w1+(89)2d1d2+89(l1d2+w1d1)]
=[(99)2−892]d1d2+10(l1d2+w1d1)
=188×10×d1d2+10×10
=188×10×10+100
=18800+100
=18900