General term of ap whose 7th term is -1 and 16th term is 17
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a7 = -1
a + 6d = -1............ ( i )
a16 = 17
a + 15d = 17.............( ii )
on equating i and ii
9d = -18
d = -2
putting value of d I'm eq i
a + 6d = -1
a + 6(-2) = -1
a + (-12) = -1
a - 12 = -1
a = -11
an = a + (n-1)d
17 = -11 + (n -1)-2
28 = (n-1) -2
28/-2 = n-1
-14= n-1
-13 =n
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