The sum of first,third,and seventh term of an ap is 216.Find the sum of the first 13term
vishnupriyacbe23:
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t1=a+ d
t3=a+2d
t7=a+6d
t1 +t3 +t7=216
a+d+a+2d+a+6d= 216
3a+9d=216
a+ 3d=72...(1)
t13=a + 12d
so, from (1 )we get
t13 is 4 times of the sum of 1st 3rd nd 7th term
therefore,
a+12d =72×4
, =288
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