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Please talk about the second window of Unit 2 in the second volume of the fourth grade of Mathematics Su Education Edition [understanding of angles and triangles]
The sum of the internal angles of a triangle is 180 degrees.

A triangle with three acute angles is an acute triangle.

A triangle with right angles is a right triangle.

A triangle with an obtuse angle is an obtuse triangle.

The sum of two sides of a triangle is greater than the third side.

A triangle with two equal angles is an isosceles triangle

A triangle with three equal sides is an equilateral triangle