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Q9:
If the 3rd and the 9th terms of an AP are 4 and – 8 respectively, which term of this AP is zero?

Solution :

Given:

The 3rd term of an Arithmetic Progression ($a_3$) = $4$.

The 9th term of an Arithmetic Progression ($a_9$) = $-8$.

To find:

The value of $n$ such that the $n^{th}$ term ($a_n$) = $0$.


Step 1: Defining the variables and the general formula.

Let the first term of the Arithmetic Progression be $a$ and the common difference be $d$.

The formula for the $n^{th}$ term of an AP is given by: $a_n = a + (n - 1)d$.


Step 2: Formulating equations based on the given terms.

For the 3rd term ($n=3$):

$a_3 = a + (3 - 1)d = 4$

$a + 2d = 4$ --- (Equation 1)

For the 9th term ($n=9$):

$a_9 = a + (9 - 1)d = -8$

$a + 8d = -8$ --- (Equation 2)


Step 3: Solving the system of linear equations.

Subtract Equation 1 from Equation 2 to eliminate $a$:

$(a + 8d) - (a + 2d) = -8 - 4$

$a - a + 8d - 2d = -12$

$6d = -12$

$d = \frac{-12}{6}$

$d = -2$


Step 4: Finding the first term ($a$).

Substitute $d = -2$ into Equation 1:

$a + 2(-2) = 4$

$a - 4 = 4$

$a = 4 + 4$

$a = 8$


Step 5: Determining the term that equals zero.

We set $a_n = 0$ and solve for $n$ using the formula $a_n = a + (n - 1)d$:

$0 = 8 + (n - 1)(-2)$

Subtract 8 from both sides:

$-8 = (n - 1)(-2)$

Divide both sides by $-2$:

$\frac{-8}{-2} = n - 1$

$4 = n - 1$

$n = 4 + 1$

$n = 5$


Final Answer: The 5th term of this AP is zero.


More Questions from Class 10 Mathematics Arithmetic Progression EXERCISE 5.2


CBSE Solutions for Class 10 Mathematics Arithmetic Progression


Chapters in CBSE - Class 10 Mathematics


Other Subjects in CBSE - Class 10

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