Question

how many term of the sequence 18, 16, 14, ....shuld bhi taken so that their sum is 78? explain double answer

Answer 1

Both answers are correct as :-

AP : 18, 16, 14, 12
Common difference,d = 16-18= -2
Sn= 78
n/2{2a+(n-1)d}=78
n/2{2(18)+(n-1)(-2)}=78
n/2{36-2n+2}=78
n/2{38-2n}=78
19n - n sq. = 78
-n square + 19n - 78 =0
n(-n+13)-6(n+13)=0
(-n+13)(n-6)=0
-n+13=0 or n-6=0
n=6,13

hope this helps you out!

Answer 2

hair the common difference is negative therefor terms gieson deamination and the 12th and 6 terms become the same