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Finding number of terms given s
Finding number of terms given s
Last updated at March 19, 2021 by Teachoo
Example 12 If the sum of the first 14 terms of an AP is 1050 and its first term is 10, find the 20th term. Here, Sum of the first 14 terms of an AP is 1050. So, S14 = 1050 Also, a = 10 & n = 14 We know that Sum = š/š[šš+(šāš)š ] Putting values 1050 = 14/2[2Ć(10)+(14ā1)š] 1050 = 7[20+13š] 1050/7=20+13š 150 = 20 + 13d 13d = 150 ā 20 13d = 130 d = 10 We have to find 20th term We use the formula an = a + (n ā 1) d Putting a = 10 , d = 10 , n = 20 a20 = 10 + (20 ā 1) 10 a20 = 10 + (19 )Ć10 a20 = 10 + 190 a20 = 200