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Question
Find the sum given below:
–5 + (–8) + (–11) + ... + (–230)
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Solution
–5 + (–8) + (–11) + ... + (–230)
For this A.P.,
a = −5
l = −230
d = a2 − a1
= (−8) − (−5)
= − 8 + 5
= −3
Let −230 be the nth term of this A.P.
l = a + (n − 1)d
−230 = − 5 + (n − 1) (−3)
−225 = (n − 1) (−3)
(n − 1) = 75
n = 76
And Sn = `n/2(a+1)`
= `76/2[(-5)+(-230)]`
= 38 × (-235)
= -8930
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