Question

sum of the n turm of an ap is 54 . its first term is 21 and common difference is -3 justify the answer for two values of n

Answer 1

Answer:

Step-by-step explanation:

Sₙ=54

a=21

d= -3

Sₙ=n/2*(2a+(n-1)d)

54=n/2*(42-3n+3)

108=n(45-3n)

45n-3n²=108

3n²-45n+108=0

3n²-36n-9n+108=0

3n(n-12)-9(n-12)=0

(3n-9)(n-12)=0

n=9/3,12

n=3,12