If the inscribed circle of triangle ABC and the tangent points of three sides are D, e and f respectively, the triangle def must be an acute triangle Why?

If the inscribed circle of triangle ABC and the tangent points of three sides are D, e and f respectively, the triangle def must be an acute triangle Why?

In triangle ABC, if the arc of angle a is a semicircle or an arc larger than semicircle, the two edges of angle a form two parallel lines or two radiation lines. Therefore, the arc of angle a can only be an arc smaller than semicircle. Then, the angle of triangle def opposite to angle a can only be an acute angle less than 90 degrees, The other two angles of triangle def can only be acute angles. The proposition is true