Question

Which term of the A.P :21,42,63,84.......is 210​

Answer 1

Answer:

a = 21 and d = 42 - 21 = 21 and an = 210

using formula for this equation

an = a + ( n - 1 ) × d

210 = 21 + ( n - 1 ) × 21

210 = 21 + 21n - 21

210 = 21n

21n = 210

n = 210/21

n = 10

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Answer 2

$\huge\tt{\bold{\underline{\underline{Question᎓}}}}$

Which term of the A.P :21,42,63,84.......is 210

$\huge\tt{\bold{\underline{\underline{Answer᎓}}}}$

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Formula used here :

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

Here,an =210

a=21

A2=42

D=A2-a1

=42-21=21

D=21

$= > 210 = 21 + (n - 1)21$

$= > 210 = 21 + 21n - 21$

$= > 210 = 21n$

$= > n = \frac{210}{21} = 10$

Therefore,10th term of the A.P is 210

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

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

When last term given:

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

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