in an A.P. 7th term is zero show that 15th term is twice the 11th term
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According to 1 condition ,
tn = a + ( n-1 ) d
put n = 7
t7 = a + ( 7-1 ) d
0 = a + 6d......... 1
According to 2 condition,
t15 = 2* t11
a + ( 15-1 ) d = 2* a ( 11-1 )d
a + 14d = 2* (a + 10d)
a + 14d = 2a + 20d
a - 2a = 20d - 14d
-a = 6d
a = -6d
a + 6d = 0........ 2
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