Question

Find the sum of the first 18 terms of the A.P is 12,8,4,....
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Answer 1

Step-by-step explanation:

12,8,4,....…………………………

t1=12, t2=8, t3=4.……………………

d=t2-t1

d=8-12

d=-4

t18=?

by using formula,

tn=a+(n-1)d

t18=12+(18-1)×(-4)

t18=12+17×(-4)

t18=12-68

t18=-56

now sum of 18 terms,

S18=?

by using formula,

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

2

S18=18[2×12+(18-1)×(-4)]

2

S18=9[24+17×(-4)]

S18=9(24-68)

S18=9×(-44)

S18=-396

the sum of 18 th term of an a.p. is -396.