How Many Triangles?
A Method to count:
Assume first that the smallest triangles are of a unit length. Note that every possible triangle are equilateral.
Now, count as follows:
(i) How many triangles are of a unit length?
9 unit-length triangles
(ii) How many triangles are of two unit lengths?
3 two-unit-length triangles
(iii) How many triangles are of three unit lengths? 1 three-unit-length triangle
So, in total there are:
9 + 3 + 1 = 13 (Answer) triangles in all.
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