True or false: angles between lines

Exercise and reference answers

Questions

1. Statement is true

   Yes

   Statement

   cos θ = |l₁l₂ + m₁m₂ + n₁n₂| gives the acute angle between two lines.

2. Statement is true

   No

   Statement

   The modulus can be dropped without changing the answer.

3. Statement is true

   Yes

   Statement

   The angle between two skew lines is found using parallel lines through a point.

4. Statement is true

   Yes

   Statement

   Two lines with direction cosines l₁l₂ + m₁m₂ + n₁n₂ = 0 are perpendicular.

5. Statement is true

   No

   Statement

   The angle formula needs the lines to actually intersect.

Explanation