Question

Find the sum of the first 20 terms of the arithmetic progression 3, 7, 11, 15, ...

• A. 820
• B. 800
• C. 840
• D. 780

Answer 1

The Correct Option is A

Step 1: Here \(a=3,\ d=4,\ n=20\). Sum \(= \dfrac{n}{2}\left[2a+(n-1)d\right]\).
Step 2: \(= \dfrac{20}{2}\left[6 + 19\times4\right] = 10\times82 = 820\).

Quick Tip: AP sum \(=\dfrac{n}{2}[2a+(n-1)d]\).