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Question:
In an A.P , if S₅ + S₇ = 167 and S₁₀ = 235 . Then find the AP where Sₙ denotes the sum of first n terms .
Answer:
1 , 6 , 11 ...
Step-by-step explanation:
Let the first term of the AP be a .
Let the common difference of the AP be d .
We know that the sum of n terms is given by the formula :
Sn = n/2 [ 2 a + ( n - 1 ) d .
Hence sum of 10 terms will be :
S = 10/2 [ 2 a + ( 10 - 1 ) d ]
⇒ 235 = 5 [ 2 a + 9 d ]
⇒ 2 a + 9 d = 235/5
⇒ 2 a + 9 d = 47 --------(1)
S₅ + S₇ = 167
⇒ 5/2 [ 2 a + 4 d ] + 7/2 [ 2 a + 6 d ] = 167
⇒ 5/2 × 2 ( a + 2 d ) + 7/2 × 2 ( a + 3 d ) = 167
⇒ 5 ( a + 2 d ) + 7 ( a + 3 d ) = 167
⇒ 5 a + 10 d + 7 a + 21 d = 167
⇒ 12 a + 31 d = 167 -------(2)
Multiplying equation (1) by 6 we get :
⇒ 12 a + 54 d = 282 -----(3)
Subtracting equation (3) from (2) we get :
⇒ 54 d - 31 d = 282 - 167
⇒ 23 d = 115
⇒ d = 115/23
⇒ d = 5
Putting the value of d in equation 1 we get :
⇒ 2 a + 9(5) = 47
⇒ 2 a + 45 = 47
⇒ 2 a = 47 - 45
⇒ 2 a = 2
⇒ a = 2/2
⇒ a = 1
A.P = 1 , 1 + 5 , 1 + 10