Question

Mark Y inside regular pentagon PQRST, so that PQY is equilateral. Is RYT straight? Explain.

Answer 1

no, because of the 4-3 theorem

Answer 2

What is the 4-3 theorem ....

Answer 3

The 4-3 theorem is a troll theorem showing that 3=4.

To answer the question...
This is provable with some geometry.

Let us suppose there is a regular pentagon PQRST with a point Y inside it such that PQY is an equilateral triangle. Then, each angle of triangle PQY is 60 degrees.
Let us also suppose the line RYT is straight. Thus, the angle RYT is 180 degrees. Then the triangle RST must be isosceles. Since we know the angles of a regular pentagon are 108 degrees, angle RST must be 108 degrees. Then angles RTS and TRS must be 36 degrees.