Answer: 1/(1-x)².
- A 1/(1-x)²
- B 1/(1-x)
- C x/(1-x)²
- D 1/(1-x) + 1
Correct answer: A. 1/(1-x)²
Explanation: This is the derivative form: d/dx [sum xn] = sum n xn-1, so sum (n+1)xn = 1/(1-x)². Replacing n+1 with the lead: sum from n=1 is (1-x)-2 = 1/(1-x)².
Bar heights for AP 2, 5, 8, 11 with the common difference d=3 marked between consecutive terms.
Concept context
Arithmetic progressions, geometric progressions, and sums