Question

Which term of an A.P 16, 11,6 ,1 ,.... Is -54.?

Answer 1

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Which term of an A.P 16, 11,6 ,1 ,.... Is -54.?

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Let 'n' be the term of -54

given : a=16

A2=11

D=A2-a1

=11-16=-5

D=-5

formula used here :

$\bold{\boxed{an = a + (n - 1)d}}$

$an = 16 + (n - 1) - 5$

$= > - 54 = 16 - 5n + 5$

$= > - 54 = 21 - 5n$

$= > - 54 - 21 = 5n$

$= > - 75 = 5n$

$= > n = - 15$

$\huge\mathcal{Other\: Formulas:}$

$\bold{\boxed{sn = \frac{n}{2} (a + (n - 1)d}}$

sum formula when last term given:

$\bold{\boxed{sn = \frac{n}{2} (a + l)}}$

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