Find the sum of all two-digit odd positive numbers.
Step-by-Step Solution
Key Concept: Two digit odd numbers: $11, 13, 15, \dots, 99$. $a = 11, d = 2, l = 99$. $99 = 11 + (n - 1)2 \Rightarrow (n - 1)2 = 88 \Rightarrow n = 45$. $S_{45} = \dfrac{45}{2}(11 + 99) = 45 \times 55 = 2475$.
$99 = 11 + (n - 1)2 \Rightarrow n = 45$. [1.0 Mark]
$S_{45} = \dfrac{45}{2}(11 + 99) = 45 \times 55 = 2475$. [1.0 Mark]
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🎯 Official CBSE Marking Scheme:
Finding $n = 45$: 1.0 Mark
Evaluating sum $= 2475$: 1.0 Mark
Correct Answer: