Question

The number of terms of the A.P: 24, 21, 18 ...... must be taken so that their sum is 78

2 points

n = 4 or 13

n = 5 or 12

n = 6 or 11


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Answer 1

⭐Answer⭐

➡13 terms

Solution :

Given :

➡a = first term = 24

➡d= common difference = - 3

➡Sum of the terms = 78

To find:

Number of terms so that the sum of the following Arithmetic progression will be 78

Sum of n terms of an A.P =(n/2) (2a+(n-1)d)

Substituting the values,

n/2× (2 × 24 + (n - 1) - 3 = 78

=> 78×2 = n (2×24+(n-1)-3)

=> 156 = n (48-3n+3)

=> 156 = n (51-3n)

=> 156 = 51n - 3n²

=> -3n²+51n-156

=> -3n²+39n+12n-156

=> -3(n-13)+12(n-13)

=> n-13=0

=> n = 13

13 terms shall be considered to get the sum as 78

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Answer 2

Answer:

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