Which term of A.P: -2, -7, -12... Will be -77? Also find the sum of this AP up to term-77
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GIVEN
a = -2
d = -5
One of the Term (an = -77)
TO FIND :- n and sum of n terms.
SOLUTION
an = a + (n - 1)d
=> - 77 = -2 + (n - 1) -5
=> - 75 = (n - 1) -5
=> 15 = n - 1
=> n = 16
Hence 14th Terms will be -77.
Now
sum of 16th term
= n/2[2a + (n-1)d]
= 8[ - 4 + 15 × -5]
= 8[ -4 -75]
= 8 × (-79)
= -632(ANS)
Hence the sum is -632
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