Another way :

You can see each set as a triangle that is half of a parallelogram.

Put two triangles together to get the parallelogram and you can form rows of circles.

Eg. First set

If you join two identical triangles together you get :

○○○/○
○○/○○
○/○○○

This is a parallelogram with 3 rows of 4 circles. Total circles = 3 x 4 = 12

First set is only half of that
= 3 x 4 ÷ 2
= 12 ÷ 2
= 6



Likewise for the rest,

Second set = 4 x 5 ÷ 2 = 20 ÷ 2 = 10
Third set = 5 x 6 ÷ 2 = 30 ÷ 2 = 15

The number of circles per row is always 1 more than the number of rows.

The number of rows is always 2 more than the set number.

So for 10th set,

Number of rows = 10 + 2 = 12
Number of circles per row = 12 + 1 = 13

Total

= 12 x 13 ÷ 2
= 78


For 35th set,

Number of rows = 35 + 2 = 37
Number of circles per row = 37 + 1 = 38

Total

= 37 x 38 ÷ 2
= 703

The last row for 35th set has 37 circles since the pattern starts from 3 circles in the first set.

Yes. Forgot to add 2 on 35...
Working = 37x38 /2 .......

Thank you v much !

Thank you