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CBSE Class 10 Answered

The difference between sum of first 20 terms and sum of first 12 terms of an AP is 1420.if 7th term of the AP is 73,find it's 27 th term
Asked by anilavthomas | 17 Jan, 2019, 07:10: AM
answered-by-expert Expert Answer
Let a be the first term and d be the common difference of A.P.
 
Sum of 20 terms = S20 = (20/2)[ 2a+19d ] = 20a + 190d
Sum of 12 terms = S12 = (12/2)[ 2a+11d ] = 12a + 66d
 
S20 - S12 = 8a +124d = 1420   or  2a+31d = 355 .................(1)
 
7th term = t7 = a + 6d = 73 ..............................(2)
 
By solving eqns.(1) and (2), we get a = 7 and d = 11
 
27th term = t27 = 7 + (26×11) = 293
Answered by Thiyagarajan K | 17 Jan, 2019, 10:35: AM

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