Arithmetic progression (nth term): Find the 21st term of 3, 9, 15, ... Sequence parameters: first term a = 3; common difference d = 6. Compute t(21) using the AP formula. Choose the correct value.

Q: Arithmetic progression (nth term): Find the 21st term of 3, 9, 15, ... Sequence parameters: first term a = 3; common difference d = 6. Compute t(21) using the AP formula. Choose the correct value.

Given a = 3, d = 6, n = 21 Formula t(n) = a + (n−1)d Step-by-step calculation t(21) = 3 + 20×6 = 3 + 120 = 123 Verification Terms grow: 3, 9, 15, ... adding 6 each time; 21st fits 123.

Difficulty: Easy

117
121
123
129

Correct Answer: 123

Explanation:

Given
a = 3, d = 6, n = 21

Formula
t(n) = a + (n−1)d

Step-by-step calculation
t(21) = 3 + 20×6 = 3 + 120 = 123

Verification
Terms grow: 3, 9, 15, ... adding 6 each time; 21st fits 123.

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