Question

Which term of the AP : 3 , -2 , -7 , -12 ...... Will be -77 ? Find the sum of this A.P up to the term -77? T

Answer 1

17th term will be the -77

Answer 2

a1=3
a2=-2
a3=-7

(common difference)d=a2-a1

◆d=-2-3=-5◆

Use formula

an=a+(n-1)d

-77=3+(n-1)(-5)
-80=(n-1)(-5)

n-1=16
n=17

◆17th term of this A.P. is -77◆

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

Sn=17/2[2×3+(17-1)(-5)

Sn=17/2[6+(-80)]

Sn=17×(-37).

Sn=-629.