← Complete problem index

PROJECT EULER · #0299

Three Similar Triangles

Statement only · SolvedOriginal problem ↗

Four points with integer coordinates are selected:
A(a,0), B(b,0), C(0,c) and D(0,d), with 0<a<b and 0<c<d.
Point P, also with integer coordinates, is chosen on the line AC so that the three triangles ABP, CDP and BDP are all similarHave equal angles.

0299_ThreeSimTri.gif

It is easy to prove that the three triangles can be similar, only if a=c.

So, given that a=c, we are looking for triplets (a,b,d) such that at least one point P (with integer coordinates) exists on AC, making the three triangles ABP, CDP and BDP all similar.

For example, if (a,b,d)=(2,3,4), it can be easily verified that point P(1,1) satisfies the above condition. Note that the triplets (2,3,4) and (2,4,3) are considered as distinct, although point P(1,1) is common for both.

If b+d<100, there are 92 distinct triplets (a,b,d) such that point P exists.
If b+d<100000, there are 320471 distinct triplets (a,b,d) such that point P exists.

If b+d<100000000, how many distinct triplets (a,b,d) are there such that point P exists?

Write-up coming later

The complete problem is available here. An approach, code, and answer will be added later.