Mathematics
The 5th term of an A.P. is −3 and its common difference is −4. The sum of its first 10 terms is :
−50
−40
−60
−30
AP
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Answer
Given,
a5 = -3
d = -4
n = 10
We know that,
⇒ an = a + (n - 1)d
⇒ a5 = a + (5 - 1)(-4)
⇒ -3 = a + (4)(-4)
⇒ -3 = a - 16
⇒ a = 16 - 3
⇒ a = 13.
We know that,
⇒ Sn = [2a + (n - 1)d]
⇒ S10 = [2(13) + (10 - 1)(-4)]
= 5[26 + (9)(-4)]
= 5[26 - 36]
= 5(-10)
= -50.
Hence, option 1 is the correct option.
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