Question

15, 10, 5, . . . In this A.P. sum of first 10 terms is . . .
(A) -75
(B) -125
(C) 75
(D) 125

Answer 1

a=15         n=10
d= 10-5
  = -5
 d= -5
    
Sn= n/2{2a+ (n-1)d}

    =  5{ 30 + 9(-5)}
    =  5{-15}
  
    =  -75

Answer 2

Hey friend,

Here is your answer,
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First term (a) = 15

Common difference (d) = -5

Number of terms (n) = 10


Sn =  n  [2a+(n-1)d]
          2


S₁₀ = 10 * [2*15 + (10-1)*(-5)]
           2

S₁₀ = 5 * [30 + 9*(-5)]

S₁₀ = 5 * [30 - 45]

S₁₀ = 5 * (-15)

S₁₀ = -75

Hence, option (A) -75 is the correct answer.
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