NCERT Exemplar Solutions
Class 6 - Mathematics - Unit 3: Integers - Problems and Solutions

Question 72

Question. 72

72. Observe: \(1+2-3+4+5-6-7+8-9=-5\). Change one ‘−’ sign as ‘+’ sign to get the sum \(9\).

Answer:

Change “−7” to “+7”.

Detailed Answer with Explanation:

1. Start with the given sum:
   (1+2-3+4+5-6-7+8-9=-5)
2. We want the new sum to be (9).
   Difference needed: (9-(-5)=14).
3. Changing one term from a minus to a plus increases the total by twice that number.
   Reason: (-k) becomes (+k).
   Increase (=;(+k)-(-k)=2k).
4. We need an increase of (14). So solve (2k=14).
   (k=7).
5. Change (-7) to (+7).
6. Check the new sum quickly:
   (1+2-3+4+5-6+7+8-9)
   (=(1+2)=3)
   (3-3=0)
   (0+4=4)
   (4+5=9)
   (9-6=3)
   (3+7=10)
   (10+8=18)
   (18-9=9).
7. Final sum (=9). Done.

ChatGPT can make mistakes. Check important info. See Cookie Preferences.