A plane figure with three straight sides and three angles.
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One of the triangle's angles is 90 degrees (a rightangle) and the two others are both 45 degrees. The three angles in a triangle must always (and will) add up to 180 degrees.
The basis of symmetry is that everything on one side of an imaginary line is the same as the other. This means that if you split a triangle down the middle by an imaginary line, one side of the triangle on both halves will be the same. This defines an isosceles triangle. An equilateral triangle also has line symmetry.
This is a true statement that defines a triangle in geometry. However, there are many different types of triangles, such as obtuse, acute, scalene, right, equilateral, and isosceles triangles.
In a scalene triangle, the angle sum is still 180 degrees. What defines a scalene triangle is that it has three sides with different lengths, and therefore, also different angles. It can be a right triangle for example, with the angles 30, 90, and 60 degrees. It's angle sum always sums up to 180 as in all triangles. Note: It doesn't have to be a right triangle. It can be any kind of triangle as long as it has three different angles and sums up 180 degrees.
Yes. The square is a special form of the angle notation, which specifies a right angle. It is common to put it there, since it clearly defines that triangle as a right-angled and not just a very close one.