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Sum of the first twenty terms of an ap is -240 and its first term is 7. Find it's 24th term

Posted by Fai Tabs_246 4 years, 2 months ago

CBSE > Class 10 > Mathematics

1 answers

Sanskriti Pancheswer 4 years, 2 months ago

Given : Sum of 20 terms of an AP =−240 and First term 7 Sum of first n terms of an AP is given as S= 2 n [2a+(n−1)d] n=20,S=−240,a=7 ⇒−240= 2 20 [2×7+(20−1)d] ⇒−240=10[14+19d] ⇒−240=140+190d ⇒−190d=140+240 ⇒−190d=380 ⇒d= −190 380 =−2 n th term of an AP =a n =a+(n−1)d ⇒a 24 =7+(24−1)(−2) ⇒a 24 =7−46 ⇒a 24 =−39

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