triangle, then a square, so the pattern is more or less ▲ ■ ▼ ■ ▲ ■ ▼ ■ ▲ ■ ▼ ■ and so on.
notice, the first is the triangle, the second is the square, so all ODD integers, 1,3 5, 7 and so on, will be triangles.
and all EVEN integers will be squares, so, the 50th shape is, well, you already know.
