Find the sum of : 7 + 101/2 + 14 +..........+ 84
Dear student,
a=14/2
d = 7/2
an = 84
an = a+(n-1)d
84 = 7 + (n-1)7/2
77 = (n-1)7/2
n-1 = 22
n=23
s23 = 23/2 {2a+(n-1)d}
s23 = 23/2 {2*7 +22*7/2}
s23 = 23/2(14+77)
s23 = 1046.5
We can't take 84 as Sn because it is the last term of an AP. Last term of an AP is not the sum. It is the sum that we need to calculate according to the question.
Hope, this clears your doubt.
Regards
a=14/2
d = 7/2
an = 84
an = a+(n-1)d
84 = 7 + (n-1)7/2
77 = (n-1)7/2
n-1 = 22
n=23
s23 = 23/2 {2a+(n-1)d}
s23 = 23/2 {2*7 +22*7/2}
s23 = 23/2(14+77)
s23 = 1046.5
We can't take 84 as Sn because it is the last term of an AP. Last term of an AP is not the sum. It is the sum that we need to calculate according to the question.
Hope, this clears your doubt.
Regards