If the sum of the first (r) natural numbers is (153), what is the value of (r+2)?
Answer and explanation
Correct answer: 19
The sum of the first r natural numbers is \\(\frac{r(r+1)}{2}\\). Thus, \\(\frac{r(r+1)}{2}=153\\), so \\(r(r+1)=306=17\times18\\) and \\(r=17\\). Therefore, \\(r+2=17+2=19\\). Option 18 is only the value of \\(r+1\\), not \\(r+2\\). Exam tip: when a sum is given, first use \\(\frac{n(n+1)}{2}\\) to find n.
Frequently asked questions
What is the correct answer to this question?
19
Why is this the correct answer?
The sum of the first r natural numbers is \\(\frac{r(r+1)}{2}\\). Thus, \\(\frac{r(r+1)}{2}=153\\), so \\(r(r+1)=306=17\times18\\) and \\(r=17\\). Therefore, \\(r+2=17+2=19\\). Option 18 is only the value of \\(r+1\\), not \\(r+2\\). Exam tip: when a sum is given, first use \\(\frac{n(n+1)}{2}\\) to find n.
Which subject and chapter does this question cover?
This is a Class 9 Mathematics question. Chapter: Sequences and Progressions. Topic: Sum of first n natural numbers.
Student feedback
Was this question useful?
👍 0 Helpful 👎 0 Not helpful
Yes 0% No 0%
0 responsesStudent Reviews
No published reviews yet.
