UPSC NDA I 2017 Mathematics Solved Paper

Given: Sum of n terms of an AP=n2−2n .
To find: The fifth term = ?
Sum of ' n ' terms of an AP=n2−2n
∴ Sum of 1‌st ‌5 terms
⇒S5=52−2⋅(5)
‌‌=25−10=15
Similarly,
Now, Sum of first 4 terms
S4=42−2⋅(4)
‌‌=16−8=8
∴ The 5 ^th term of an AP
⇒T5=S5−S4‌‌.. .(Using‌Tn=Sn−Sn−1)
‌‌=15−8
‌‌=7