It can be observed that the number of trees planted by the students is in an AP.
\(1, 2, 3, 4, 5, …..,12\)
First term, \(a = 1\)
Common difference, \(d = 2 − 1 = 1\)
\(S_n = \frac {n}{2}[ 2a + (n-1)d]\)
\(S_{12} = \frac {12}{2}[ 2(1) + (12-1)1]\)
\(= 6 (2 + 11)\)
\(= 6 (13)\)
\(= 78\)
Therefore, number of trees planted by \(1\) section of the classes \(=\) \(78\)
Number of trees planted by \(3\) sections of the classes = \(3 × 78 = 234\)
Therefore, \(234\) trees will be planted by the students.