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## 'Isosceles Triangles' printed from http://nrich.maths.org/

Yanqing from Devonport High School for Girls
sent us the only correct solution to this problem. She wrote:

First, I worked out how many types of isosceles triangles there are
that have an area of 9cm², and can be drawn with integer
co-ordinates.

I found 3 types:

base = 18, height = 1,

base = 6, height = 3,

base = 2, height = 9.

(These would not work the other way round because it would be
impossible to have integer co-ordinates.)

I then worked out that because there are 3 vertices on a triangle,
and there are 4 directions in which a triangle could go, it would
be possible to have 12 ways of placing one type of triangle around
the point (20,20): you can place it 3 ways upwards, 3 ways to the
left, to the right and down.

I then tried this out and found it to be the case.

Because there are 3 types of isosceles triangles, and 12 ways to
place each one, it is possible to draw 36 isosceles triangles with
integer co-ordinates and one vertex on (20,20) which have an area
of 9cm².

Well done Yanqing - this is a very clear
explanation.