Solution:
It is clear that the number of trees planted by students is in the form of A.P.
1, 2, 3, 4, 5………………..12
The first term, a = 1
The common difference, d = 2−1 = 1
Sn = n/2 [2a +(n-1)d]
S12 = 12/2 [2(1)+(12-1)(1)]
= 6(2+11)
= 6(13)
= 78
Therefore, trees planted by 1 section of the classes = 78
Trees planted by 3 sections of the classes = 3×78 = 234
As a result, in total 234 trees will be planted by the students.