The sum of the first n odd numbers is n². Use this rule to find 1 + 3 + 5 + … + 19.

100. 19 is the 10th odd number (2 × 10 − 1 = 19), so n = 10 and the sum is 10² = 100. (361 wrongly uses 19².)

The sum of the first $n$ odd numbers is $n^{2}$ . Use this rule to find 1 + 3 + 5 + … + 19.

Class 7Formulas and Rules from PatternsAlgebra

19 is the 10th odd number

(2 \times 10 - 1 = 19)

, so

n = 10

and the sum is

1 0^{2} = 100

. (361 wrongly uses

1 9^{2} .

)

Answer: A) 100

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