Angle between two lines - Direction ratios or cosines
Angle between two lines - Direction ratios or cosines
Last updated at April 16, 2024 by Teachoo
Ex 11.2, 2 Show that the line through the points (1, −1, 2), (3, 4, −2) is perpendicular to the line through the points (0, 3, 2) and (3, 5, 6). Two lines with direction ratios 𝑎1, 𝑏1, 𝑐1 and 𝑎2, 𝑏2, 𝑐2 are perpendicular to each other if 𝒂𝟏 𝒂𝟐 + 𝒃𝟏 𝒃𝟐 + 𝒄𝟏 𝒄𝟐 = 0 Now, a line passing through (x1, y1, z1) and (x2, y2, z2) has the direction ratios (x2 − x1), (y2 − y1), (z2 − z1) A (1, −1, 2) B (3, 4, −2) Direction ratios (3 − 1), 4 − (−1), −2 − 2 = 2, 5, –4 ∴ 𝒂𝟏 = 2, 𝒃𝟏 = 5, 𝒄𝟏 = −4 C (0, 3, 2) D (3, 5, 6) Direction ratios (3 − 0), (5 − 3), (6 − 2) = 3, 2, 4 ∴ 𝒂𝟐 = 3, 𝒃𝟐 = 2, 𝒄𝟐 = 4 Now, 𝒂𝟏 𝒂𝟐 + 𝒃𝟏 𝒃𝟐 + 𝒄𝟏 𝒄𝟐 = (2 × 3) + (5 × 2) + (−4 × 4) = 6 + 10 + (−16) = 16 − 16 = 0 Therefore the given two lines are perpendicular.