Explanation:
To solve 3D Pythagoras and trigonometry problems, identify and sketch the key 2D right-angled triangle inside the 3D shape. For a cuboid with dimensions l, w, and h, the floor diagonal is √(l2 + w2) and the 3D space diagonal can be found in one step using d = √(l2 + w2 + h2).
The angle between a line (such as a space diagonal or slant edge) and a horizontal plane (the base) is always the angle between the line and its projection (shadow) on the base directly underneath it.
Example 1: A cuboid ABCD.EFGH measures 12 cm by 9 cm by 8 cm (vertical height). Find space diagonal AG and its angle with the base ABCD (1 d.p.).
Solution:
Base diagonal AC = √(122 + 92) = √225 = 15 cm
Space diagonal AG = √(152 + 82) = √289 = 17 cm
In ΔACG: tan(∠GAC) = 815 ⇒ ∠GAC = 28.072...°
Answer: AG = 17 cm, angle = 28.1°
Example 2: A square-based pyramid has base side 10 cm and slant edges 13 cm. Find its vertical height h (3 s.f.).
Solution:
Base diagonal = √(102 + 102) = √200 cm
Half diagonal (corner to M) = √2002 = √50 cm
h = √(132 − (√50)2) = √(169 − 50) = √119
Answer: h = 10.9 cm (3 s.f.)