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Solution


Q.

The first term of an AP is 5 and the last term is 45. If the sum of all the terms is 400, the number of terms is:
1 20  
2 8  
3 10  
4 16  

Ans.

(4)

We havea = 5, aₙ = 45 = l and Sₙ = 400∴Sn=n/2(a+l)⇒ 400=n/2(5+45)⇒ 400=n/2×50⇒ 25n=400∴ n=400/25⇒ n = 16