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sqdancefan
The Law of Cosines is useful for this.
  d² = e² + f² - 2ef·cos(D)
  cos(D) = (e² + f² - d²)/(2ef) = (1.8² + 3.1² - 2²)/(2·1.8·3.1)
  D = arccos(8.85/11.16)
  D ≈ 37.5°
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