### deltoid

• n. 三角肌
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1741, in deltoid muscle, so called for its shape, from Greek deltoeides "triangular," literally "shaped like the letter delta;" see delta + -oid.
1. Construct an animation of the deltoid sliding on two mutually perpendicular lines.

2. Show how a deltoid of maximal size slides inside a stationary astroid.

3. Construct two mutually orthogonal tangents of the deltoid. Where do they meet?

4. Construct rotating cardioid and deltoid while remaining orthogonal to each other.

5. Construct an animation displaying the deltoid sliding inside a fixed nephroid.