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Find the sum: - 5+ (-8) + (-11) +..............+ (-230)

Asked by Topperlearning User 22nd September 2017, 8:50 AM
Answered by Expert
Answer:

-5, -8, -11, … -230 forms an AP with

a = -5, d= -8 - (-5) = -3

Let -230 = an = a + (n-1)d = -5 + (n - 1)(-3)

 -230 + 5=(n - 1)(-3)

 n - 1 = begin mathsize 12px style fraction numerator negative 225 over denominator negative 3 end fraction end style 

 n - 1 = 75

 n = 76

S76 = begin mathsize 12px style straight n over 2 open parentheses straight a plus straight l close parentheses end style

= begin mathsize 12px style open parentheses 76 over 2 close parentheses end style[(-5) + (-230)]

= 38 × (-235)

= - 8930

Answered by Expert 22nd September 2017, 10:50 AM
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