Q6 of 27 Page 84

How many terms of the arithmetic sequence 5, 7, 9, …, must be added to get 140?

Using arithmetic sequence equation










⇒ n2 + 4n – 140 = 0


⇒ n2 + 14n – 10n – 140 = 0


⇒ n(n + 14) – 10(n + 14) = 0


⇒ (n + 14) (n – 10) = 0


x = –14 as we cannot have negative number of terms.


Therefore x = 10, the number of terms is 10


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