Question

1+6+11+...+x=189
Put the value of x

Answer 1

Answer:-

x=41

step by step solution:-

1. Sn =n/2{2a+(n-1)d}
2. 189=n/2{2+5n-5}. {a=1;d=5}
3. 378= n(5n-3)
4. 378=5n^2-3n
5. 5n^2- 3n-378=0
6. 5n^2-(45-42)n-378=0
7. 5n^2-45n+42n-378=0
8. 5n(n-9)+42(n-9)=0
9. (n-9)(5n+42)=0
10. n-9=0. 5n+42=0
11. n=9. n=-42/5
12. n cannot be fractional and negative, therefore n=9.
13. An= a+(n-1)d
14. x=1+(9-1)5
15. x=1+8*5
16. x=1+40
17. x=41