MCQ
The sum of first $20$ odd natural number is
- A$100$
- B$210$
- ✓$200$
- D$420$
✓
Answer
Correct option: C.
$200$
$ S _{20}=1+3+5+7+\cdots$
$\text { Here } a =1, d=2, S_0, T_{20}=( a +19 d)$
$=1+19 \times 2=38=\text { last term (l) }$
$\therefore S _{20}=\frac{ n }{2}( a + l )=\frac{20}{2}(1+39)$
$=20 \times 20$
$=400$
$\text { Here } a =1, d=2, S_0, T_{20}=( a +19 d)$
$=1+19 \times 2=38=\text { last term (l) }$
$\therefore S _{20}=\frac{ n }{2}( a + l )=\frac{20}{2}(1+39)$
$=20 \times 20$
$=400$
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