Question 30: \(\text { Find a natural number } \mathrm{n} \text { know that: } 1+2+3+\ldots+\mathrm{n}=820\)
\(\begin{aligned} &\text { We have: } 1+2+3+\ldots+n=\frac{n \cdot(n+1)}{2}=820 \\ &\Rightarrow n \ cdot(n+1)=1640=40.41 \\ &\Rightarrow n=40 \end{aligned}\)
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