Answer :

Given 4a4=18a18, a22=7

2(a+3d)=9(a+17d)


2a+6d=9a+153d


7a+197d=0


7(a+21d)=0


a+21d=0


a22=0


OR


Given: 24,21,18, ….sn=78


A=24, d=21-24=-3


Sn=n/2 [2a+(n-1)d]


78=n/2 [2(24)+(n-1)(-3)]


156=n[48-3x+3]


156=n[51-3x]


156=51n-3n2


3x2-51n+156=0


X2-17n+52=0


X2-13n-4n+52=0


n(n-13)-4(n-13)=0


(n-13)(n-4)=0


n=13, n=4


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