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Find the sum of the first 8 terms of the pattern $1, 3, 6, 10, 15, 21, l d o t s$

hard 4 marks Class 5 · Mathematics · Can You See the Pattern

Answer

120

The correct answer is 120 — So the sum is $120$

Step-by-step solution

Recognise the pattern as triangular numbers: $t_{n} = \frac{n ( n + 1 )}{2}$
Use the sum formula for the first $m$ triangular numbers: $S_{m} = \frac{m ( m + 1 ) ( m + 2 )}{6}$
Set $m = 8$ : $S_{8} = \frac{8 \times 9 \times 10}{6}$
Compute: $\frac{720}{6} = 120$
So the sum is $120$

Common mistake: Adding terms directly without using formula

Concept tested: Sum of triangular numbers

Published 4 October 2026 · How this solution was prepared and checked

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