Q8 of 50 Page 32

The sum of the first 10 terms of an arithmetic sequence is 350 and the sum of the first 5 terms is 100. Write the algebraic expression for the sequence.

S10 = 350


S5 = 100


Sn =


When n = 10,


S10 = 350 =


∴ 2a + 9d = 70 …(eq)1


S5 = 100 =


∴ a + 2d = 20 …(eq)2


Solving equations 1 and 2…


d = 6


a = 8


Hence, the sequence is 8,14,20…


an = a + (n-1)×d = 8 + (n-1)×6 = 6n + 2


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