Q.

A school has decided to plant some endangered trees on 51st World Environment Day in the nearest park. They have decided to plant those trees in few concentric circular rows such that each succeeding row has 20 more trees than the previous one. The first circular row has 50 trees.

Based on the above given information, answer the question :

Q.   If 3200 trees are to be planted in the park, then how many rows are required  


Ans.

(16)

Here a=50 and d=20

Let Sn=3200

Sn=n2[2a+(n-1)d]=3200

Sn=n2[2×50+(n-1)×20]=3200

n2+4n-320=0

(n+20)(n-16)=0

Since, n-20

  n=16

Hence, required number of rows are 16