When two straight lines are parallel, the included angle is considered as 0. When two straight lines are perpendicular, the included angle is considered as 90. When two straight lines intersect non-perpendicularly, four angles are formed, which are divided into two groups.

Two acute angles and two obtuse angles. According to the regulations, a pair of diagonal angles with acute angles are selected as the included angle between straight lines. The included angle between two straight lines refers to an angle less than or equal to 90.

According to the definition of included angle of spatial straight lines, if two spatial straight lines are parallel, the included angle is 0; If they are not parallel, including intersection and non-plane, it is an acute angle or a right angle among the angles formed by two straight lines; If two straight lines are perpendicular, it is 90; If they are not parallel or perpendicular, it is an acute angle. So the angle formed by two straight lines can't be obtuse.

The importance of learn mathematics:

1. Mathematics is closely related to our life. To say that the real effect of learning mathematics is not reflected in exam-oriented education, but in my own thinking about the future.

2. The importance of mathematics is self-evident. Mathematics is the foundation of all sciences and an important channel to cultivate logical thinking. It can be said that every major progress of our mankind is strongly supported by mathematics.

3. The application of mathematical knowledge in life is everywhere. From the calculation of daily living costs, to astronomy, geography, quality control, agricultural economy, aerospace, there are shadows of using mathematics.