A) 5600
B) 5656
C) 5680
D) 4000
Correct Answer: C
Solution :
Sum of first n natural numbers \[=(1+2+3+...+n)\] \[=\frac{1}{2}n\,\,(n+1)\] \[(45+46+47+...+114+115)\] \[=(1+2+3+...+44+45+46+...+114+115)\]\[-(1+2+3...+44)\] \[=\left( \frac{1}{2}\times 115\times 116 \right)-\left( \frac{1}{2}\times 44\times 45 \right)\] \[=(115\times 58)-(22\times 45)=(6670-990)=5680\]You need to login to perform this action.
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