That is, greater than or equal to 0 degrees and less than or equal to 90 degrees.

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.

So the included angle between the straight line and the straight line is in the range of [0,90] or [0,π/2].

Extended data:

When the slope of the straight line L does not exist, the formula of oblique section y=kx+b When k=0, y = b..

When the slope of the straight line L exists, the point inclination angle Y2-Y 1 = K (X2-X 1),

When the straight line L has a non-zero intercept on two coordinate axes, there is an intercept formula X/a+y/b= 1.

For any point on any function, its slope is equal to the included angle between its tangent and the positive direction of X axis, that is, tanα.

Slope calculation: ax+by+c=0, where k =-a/b.

Linear slope formula: k=(y2-y 1)/(x2-x 1)

The product of slopes of two vertically intersecting lines is-1:k1* k2 =-1.

When k>0, the greater the angle between the straight line and the X axis, the greater the slope; When k < 0, the smaller the angle between the straight line and the X axis, the smaller the slope.