MCQ
The sum of first $20$ odd natural numbers is :
- A$100$
- B$210$
- ✓$400$
- D$420$
✓
Answer
Correct option: C.
$400$
The frist $20$ odd natural numbers will be $\{1, 3, 5, 7, ......\}$
Here,
$a = 1$
$d = 3 - 1 = 2$
$n = 20$
$\text{S}_\text{n}=\frac{\text{n}}{2}\big[2\text{a}+(\text{n}-1)\text{d}\big]$
$\Rightarrow\text{S}_{20}=\frac{20}{2}\big[2(1)+19(2)\big]$
$\Rightarrow\text{S}_{20}=10[2+38]$
$\Rightarrow\text{S}_{20}=400$
Here,
$a = 1$
$d = 3 - 1 = 2$
$n = 20$
$\text{S}_\text{n}=\frac{\text{n}}{2}\big[2\text{a}+(\text{n}-1)\text{d}\big]$
$\Rightarrow\text{S}_{20}=\frac{20}{2}\big[2(1)+19(2)\big]$
$\Rightarrow\text{S}_{20}=10[2+38]$
$\Rightarrow\text{S}_{20}=400$
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