Matthew Fan's answer to Amy Ang's Junior College 1 H2 Maths Singapore question.
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The key is to realise u_n is an arithmetic progression
Date Posted:
4 years ago
First part actually no need to find un. Just take SN - S4
N(N+14)/3 - 4(4+14)/3
= (N² + 14N)/3 - 24
Since the sum formula already given
N(N+14)/3 - 4(4+14)/3
= (N² + 14N)/3 - 24
Since the sum formula already given
That is right, yes the solution can be made much shorter that way
thanks!
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