Question : What is the sum of the first 15 odd numbers?
Option 1: 255
Option 2: 225
Option 3: 235
Option 4: 215
Correct Answer: 225
Solution :
The first 15 odd numbers are: 1, 3, 5, 7, 9, 11, 13, 15, 17, 19, 21, 23, 25, 27 and 29
Total number of terms: 15
Applying the formula: Sum(S) = $\frac{n}{2}[2a+(n–1)d]$
Where,
$n$ = number or terms
$a$ = 1st term
$d$ = difference between two terms
So, the sum = $\frac{15}{2}[2×1+(15–1)2]$
= $\frac{15}{2}[2+(14×2)]$
= $\frac{15}{2}×30$
= 15 × 15
= 225
Hence, the correct answer is 225.
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