Answer :

The number of dots in first 3 triangles are 3, 6 and 10 respectively.

The difference between the first two terms is 6-3 = 3


And 10-6 = 4


The series continues 3, 4, 5…


Hence, this forms an AP with first term as 3 and common difference as 1


Next two terms are 4 + 1 = 5


And 5 + 1 = 6


So 4th pattern has 10 + 5 = 15 dots


5th pattern has 15 + 6 = 21 dots


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