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Arithmetic Progressions
NCERT Exemplar Ch 05
CBSE_NCERT_EXEMPLAR_CH05
Grade 10

Question:

If the $21^{\text{st}}$ term of the AP whose first two terms are $-3$ and $4$ is:

$17$
$137$
$143$
$-143$

Step-by-Step Solution

Key Concept: Here $a = -3$, common difference $d = 4 - (-3) = 7$. $a_{21} = a + 20d$.
Stepwise Solution:

$a = -3, d = 7$. [0.5 Mark]

$a_{21} = -3 + 20(7) = -3 + 140 = 137$. [0.5 Mark]

Marking Scheme:

• Finding common difference $d = 7$: 0.5 Mark
• Calculating $a_{21} = 137$: 0.5 Mark

Correct Answer: $137$
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