数学题：从距离和两个角度求高度

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Here is  a TRI  function that  I wrote  last year  for calculating the sides and
angles of right and oblique triangles.  The function is fairly easy to  use.  It
has six arguments:  Side s1, Side  s2, Side s3,  Angle a, Angle  b, and Angle c.
Angles are  in radians.   It returns  a 1  based list  so you  can use the ‘nth’
function to retrieve your results.   To display the text help  information, just
type (tri ? ? ? ? ? ?) on the command line as follows:
Note: This displays better using a courier or mono-spaced font.
Command: (tri ? ? ? ? ? ?)
tri error: (tri nil nil nil nil nil nil)
Only 2 or 3 valid arguments can be passed to tri function.
tri - Calculates the sides and angles of a triangle
Arguments: 6             |\                /\
  s1 = Side s1           |a\              /a \
  s2 = Side s2           |  \            /    \
  s3 = Side s3        s2 |   \ s3    s2 /      \ s3
  a = Angle a radians    |    \        /        \
  b = Angle b radians    |c___b\      /c________b\
  c = Angle c radians      s1              s1
Syntax example: (tri 3 ? ? 0.643501 ? ?);where ? = nil
Returns: (list nil 3.0 4.0 5.0 0.643501 0.927295 1.5708)
Note: For right triangles only supply the argument values for 2 sides,
or 1 side and 1 angle. Use ? or nil for the Angle c argument value.
For oblique triangles only supply argument values for 3 sides,
or 2 sides and 1 angle, or 1 side and 2 angles.
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; tri - Calculates the sides and angles of a triangle
; Arguments: 6             |                 /\
;   s1 = Side s1           |\               /a \
;   s2 = Side s2           |a\             /    \
;   s3 = Side s3        s2 |  \ s3     s2 /      \ s3
;   a = Angle a radians    |   \         /        \
;   b = Angle b radians    |c__b\       /c________b\
;   c = Angle c radians      s1              s1
; Syntax example: (tri 3 ? ? 0.643501 ? ?);where ? = nil
; Returns: (list nil 3.0 4.0 5.0 0.643501 0.927295 1.5708);nth 1 based list
; Note: For right triangles only supply the argument values for 2 sides, or
; 1 side and 1 angle. Use ? or nil for the Angle c argument value. For oblique
; triangles only supply argument values for 3 sides, or 2 sides and 1 angle,
; or 1 side and 2 angles.
; Programming example usages:
; (setq Side2 (nth 2 (tri 3 ? ? 0.643501 ? ?))) = 4.0
; (setq AngleB (nth 5 (tri 3 ? ? 0.643501 ? ?))) = 0.927295