1+5+9+......+81=?
Solution
Correct Answer: Option B
আমরা জানি,
শেষপদ = a + (n-1)d
সুতরাং, 81 = a + (n-1)d
⇒ 81 = 1 + (n-1)4
⇒ 81 - 1 = (n-1)4
⇒ 80 = (n-1)4
⇒ (n-1) = 80/4 = 20
⇒ n-1 = 20
∴ n = 21
এখন,
S= n[2a + (n-1)d]/2
= 21[2.1 + (21-1)4]/2
= 861
Practice More Questions on Our App!
Download our app for free and access thousands of MCQ questions with detailed solutions