We will discuss here about the condition of parallelism.

If two lines are parallel then they are inclined at the same angle θ with the positive direction of the x-axis. So, their slopes are equal.

Two lines with slopes m\(_{1}\) and m\(_{2}\) are parallel if and only if m\(_{1}\) = m\(_{2}\)

**Note:** If the slope of a line is
m then any line parallel to it will also have the slope m.

Solved examples on condition of parallelism:

**1.** Prove that the lines 3x – 2y – 1 = 0 and 9x - 6y + 5 = 0 are parallel.

**Solution:**

The slope of the lines can be found by comparing the equations with y = mx + c.

Equation of the first straight line 3x – 2y – 1 = 0

Now we need to express the given equation in the form y = mx + c.

3x – 2y – 1 = 0

⟹ -2y = -3x + 1

⟹ y = \(\frac{-3}{-2}\)x + \(\frac{1}{-2}\)

⟹ y = \(\frac{3}{2}\)x - \(\frac{1}{2}\)

Therefore, the slope (m\(_{1}\)) of the given line = \(\frac{3}{2}\)

Equation of the second line 9x - 6y + 5 = 0

Now we need to express the given equation in the form y = mx + c.

9x - 6y + 5 = 0

⟹-6y = -9x - 5

⟹ y = \(\frac{-9}{-6}\)x - \(\frac{5}{-6}\)

⟹ y = \(\frac{3}{2}\)x + \(\frac{5}{6}\)

Therefore, the slope (m\(_{2}\)) of the given line = \(\frac{3}{2}\)

Now we can clearly see that the slope of the first line m\(_{1}\) = the slope of the second line m\(_{2}\)

Therefore, the given two lines are parallel.

**2.** Find the value
of k if the lines 7y = kx + 4 and x + 2y = 3 are parallel.

**Solution:**

The slope of the lines can be found by comparing the equations with y = mx + c.

Equation of the first straight line 7y = kx + 4

Now we need to express the given equation in the form y = mx + c.

7y = kx + 4

⟹ y = \(\frac{k}{7}\)x + \(\frac{4}{7}\)

Therefore, the slope (m\(_{1}\)) of the given line = \(\frac{k}{7}\)

Equation of the second line x + 2y = 3

Now we need to express the given equation in the form y = mx + c.

x + 2y = 3

⟹ 2y = -x + 3

⟹ y = -\(\frac{1}{2}\)x + \(\frac{3}{2}\)

Therefore, the slope (m\(_{2}\)) of the given line = -\(\frac{1}{2}\)

Now according o the problem the two given lines are parallel.

i.e., m\(_{1}\) = m\(_{2}\)

⟹ \(\frac{k}{7}\) = -\(\frac{1}{2}\)

⟹ k = -\(\frac{7}{2}\)

Therefore, the value of k = -\(\frac{7}{2}\)

● **Equation of a Straight Line**

**Inclination of a Line****Slope of a Line****Intercepts Made by a Straight Line on Axes****Slope of the Line Joining Two Points****Equation of a Straight Line****Point-slope Form of a Line****Two-point Form of a Line****Equally Inclined Lines****Slope and Y-intercept of a Line****Condition of Perpendicularity of Two Straight Lines****Condition of parallelism****Problems on Condition of Perpendicularity****Worksheet on Slope and Intercepts****Worksheet on Slope Intercept Form****Worksheet on Two-point Form****Worksheet on Point-slope Form****Worksheet on Collinearity of 3 Points****Worksheet on Equation of a Straight Line**

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