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CBSE Class 10 Answered

if a times the ninth term of an ap equals 12 times the twelth term show that 21st term is zero
Asked by ABHILASHA | 16 Jan, 2019, 10:57: AM
answered-by-expert Expert Answer
It is given in the posted question : " a times of 9th term of an A.P equals 12 times of 12th term " .
I back calculated from the given answer and found that it should be " 9 times of 9th term of an A.P equals 12 times of 12th term " .
 
Hence I guess it is 9 times of 9th term
 
let a be the first term and d be the common difference os A.P
 
9(a+8d) = 12(a+11d)  
 
Hence 3a+60d = 0  or a+20d = 0
 
since a+20d is 21st term of an A.P, it is proved that 21st term is zero
Answered by Thiyagarajan K | 16 Jan, 2019, 01:22: PM

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