**Angle of inclination of a line :**

Let a straight line l intersect the x - axis at A. The angle between the positive x - axis and the line l, measured in counter clockwise direction is called the angle of inclination of the straight line l.

In the above figure, If θ is the angle of a straight line l, then we have the following important points.

(i) 0° ≤ θ ≤ 180°

(ii) For horizontal lines, θ = 0° or 180° and for vertical lines, θ = 90°

(iii) If a straight line initially lies along the x-axis and starts rotating about a fixed point A on the x-axis in the counter clockwise direction and finally coincides with the x-axis, then the angle of inclination of the straight line in the initial position is 0°and that of the line in the final position is 0°.

(iv) Lines which are perpendicular to x-axis are called as vertical lines.

(v) Lines which are perpendicular to y-axis are called as horizontal lines.

(vi) Other lines which are neither perpendicular to x- axis and nor to y-axis are called as slant lines.

The major application of angle of inclination of a straight line is finding slope.

If θ is the angle of inclination of a straight line l, then tanθ is called the slope of gradient of the line is denoted by "m".

Therefore, the slope of the straight line is

**m = tan θ**

for 0° ≤ θ ≤ 180°

Let us find the slope of a straight using the above formula

(i) For horizontal lines, the angle of inclination is 0° or 180°. That is, θ = 0° or 180°

Therefore, slope of the straight line is

m = tan0° or tan 180° = 0

(ii) For vertical lines, the angle of inclination is 90°. That is θ = 90°

Therefore, slope of the straight line is

m = tan90° = Undefined

(iii) For slant lines, if θ is acute, then the slope is positive. Whereas if θ is obtuse, then the slope is negative.

When we look at a straight line visually, we can come to know the sign of the slope easily.

To know the sign of slope of a straight line, always we have to look at the straight line from left to right.

The figures given below illustrate this.

**Problem 1 :**

Find the angle of inclination of the straight line whose slope is 1/√3.

**Solution :**

Let θ be the angle of inclination of the line.

Then, slope of the line, m = tan θ

Given : Slope = 1/√3

So, we have

tan θ = 1/√3

θ = 30°

**Hence, the angle of inclination is 30° **

**Problem 2 :**

If the angle of inclination of a straight line is 45°, find its slope.

**Solution :**

Let θ be the angle of inclination of the line.

Then, slope of the line, m = tan θ

Given : θ = 45°

Then, we have

m = tan 45°

m = 1

**Hence, the slope is 1. **

**Problem 3 :**

If the angle of inclination of a straight line is 30°, find its slope.

**Solution :**

Let θ be the angle of inclination of the line.

Then, slope of the line, m = tan θ

Given : θ = 30°

Then, we have

m = tan 30°

m = 1/√3

**Hence, the slope is ****1/√3****. **

**Problem 4 :**

Find the angle of inclination of the straight line whose slope is √3.

**Solution :**

Let θ be the angle of inclination of the line.

Then, slope of the line, m = tan θ

Given : Slope = √3

So, we have

tan θ = √3

θ = 60°

**Hence, the angle of inclination is 60° **

After having gone through the stuff given above, we hope that the students would have understood "Angle of inclination of a line".

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