Class 10 Optional Mathematics | Angle between Two Straight Lines

Optional Mathematics, Class 10

8.2 Angle between Two Straight Lines

Slopes decide the angle · Parallel and perpendicular tests

By the end of this lesson you can

📐

Find the acute and obtuse angles between two lines

🟰

Test whether two lines are parallel

⊥

Test whether two lines are perpendicular

✏️

Write parallel and perpendicular lines through a point

Derivation

From a triangle to a formula

EXTERIOR

Exterior angle of triangle EAC: θ1 = θ + θ2

SUBTRACT

Make θ the subject: θ = θ1 - θ2

TAN

Take tan and use the formula for tan(A - B)

SLOPES

Replace tan θ1 by m1 and tan θ2 by m2

The notes are free to read in full.

Presenter notes: Today we find the angle between two lines without a protractor. All we need are their slopes.