An angle greater than 0 and less than 90 is called an acute angle, an angle greater than 90 and less than 180 is called an obtuse angle, and an angle equal to 90 is called a right angle.

When the adjacent angles formed by a straight line and another horizontal straight line are equal, each of these angles is called a right angle, and this straight line is called perpendicular to another straight line. An angle smaller than a right angle is called an acute angle, and an angle larger than a right angle but smaller than a right angle is called an obtuse angle.

Extended data:

1, the sum of squares of two right angles of a right triangle is equal to the square of the hypotenuse. As shown in figure ∠ BAC = 90, then AB? +AC？ =BC？ (Pythagorean theorem)

2. In a right triangle, two acute angles are complementary. As shown in the figure, if ∠ BAC = 90, ∠ B+∠ C = 90.

3. In a right triangle, the median line on the hypotenuse is equal to half of the hypotenuse (that is, the outer center of the right triangle is located at the midpoint of the hypotenuse, and the radius of the circumscribed circle R=C/2). This property is called the hypotenuse midline theorem of right triangle.

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