% !TeX program = lualatex
% =====================================================================
%  geometry.tex
%  Geometric notation in the inline mini-language: the geometric angle,
%  parallel / perpendicular / congruent relations, vectors with the dot
%  and cross products written infix, and the named operators of plane
%  and vector geometry (collinearity, distance, midpoint, inner product,
%  orthogonal complement, orthogonal projection, scalar triple product).
%  lang=en keeps the decimal point.
% =====================================================================
\documentclass[
  margins=8,
  font=Latin Modern Roman,
  size=12,
  linespread=1.4,
  lang=en
]{scholatex}
\begin{document}

let title = <Red b 18pt c>
let h1    = <Navy b section>

<title>scholatex — geometry

% =====================================================================
<h1>Angles
% =====================================================================

The geometric angle is $angle(ABC)$, written with the hat as in the
French convention. A single point gives $angle(A)$, three points name the
angle by its vertex in the middle. A right angle reads $angle(ABC) = 90$
degrees.

<box line:Navy fill:AliceBlue radius:2 title:{A right-angled triangle}>{
In a triangle $ABC$ right-angled at $B$, the angle $angle(ABC)$ is the
right angle, while $angle(BAC)$ and $angle(BCA)$ are the two acute angles.
}

% =====================================================================
<h1>Parallel, perpendicular, congruent
% =====================================================================

Two lines are parallel, $(AB) parallel (CD)$, or perpendicular,
$(AB) perp (CD)$. The negation reads $(AB) !parallel (CD)$. Two figures
are congruent with $ABC cong A'B'C'$, similar with $ABC sim A'B'C'$.

% =====================================================================
<h1>Vectors: dot and cross products
% =====================================================================

A vector is $vec(u)$, its norm $norm(vec(u))$. The dot product is written
infix, $vec(u) . vec(v)$, and the cross product likewise, $vec(u) ^ vec(v)$.
Only a vector on both sides triggers them: $vec(u)^2$ stays a square and
$norm(vec(u))^2$ a squared norm.

<box line:Navy fill:AliceBlue radius:2 title:{Norm from the dot product}>{
The square of the norm is the dot product of a vector with itself:
$norm(vec(u))^2 = vec(u) . vec(u)$. Two vectors are orthogonal when
$vec(u) . vec(v) = 0$.
}

Collinearity reads $collinear(u, v)$, orthogonality of two vectors
$vec(u) perp vec(v)$.

% =====================================================================
<h1>Distance, midpoint
% =====================================================================

The distance between two points is $distance(A, B)$, and the midpoint of
a segment is $midpoint(A, B)$.

% =====================================================================
<h1>Higher vocabulary
% =====================================================================

The inner product of an abstract space is $inner(u, v)$, the orthogonal
complement of a subspace $ortho(F)$, and the orthogonal projection of a
vector onto it $orthogonalprojection(F, x)$. The scalar triple product of
three vectors is $triple(u, v, w)$.

<box line:Navy fill:AliceBlue radius:2 title:{The Cauchy–Schwarz inequality}>{
For any two vectors, $abs(vec(u) . vec(v)) <= norm(vec(u)) norm(vec(v))$,
with equality exactly when $collinear(u, v)$.
}

% =====================================================================
<h1>Coordinate system and plane figures
% =====================================================================

A coordinate system is $frame(O, i, j)$ in the plane, $frame(O, i, j, k)$
in space; the origin stays bare and each basis vector is arrowed. A
numbered basis works the same way: $frame(O, e_1, e_2)$.

The figure symbols read directly: a triangle $triangle(ABC)$, an arc
$arc(AB)$, a right angle $rightangle$, a parallelogram $parallelogram$.
A circle is $circle(O, r)$ by centre and radius, or $circle(A, B, C)$
through three points.

An orthonormal coordinate system is $orthoframe(O, i, j)$. An angle in
degrees reads 45° directly, or $angle(ABC) = 90°$ in a formula.

% =====================================================================
<h1>Vector components
% =====================================================================

A vector given by its components reads inline $vector(3, 5)$ or as a
column $colvec(3, 5)$; in space, $vector(1, 2, 3)$ and $colvec(1, 2, 3)$.

let u = vector(3, 5)
With $u$ stored, its first component is #{u[1]} and its squared norm is
#{u[1]*u[1] + u[2]*u[2]}.

% =====================================================================
<h1>Drawing figures: the draw block
% =====================================================================

The «draw» block draws a figure from its description alone — the author
gives the shape and the measurements, and the coordinates are computed and
placed automatically. One drawing unit is one centimetre, so a side given as
DK:6 measures 6 cm on the page. Equal sides and right angles are coded only
with marks:on; side lengths are shown with measures:cm or measures:mm. A
figure wider than the page is the cue to adjust the measurements before
printing.

An equilateral triangle of side 5, with the equal sides coded and each
length labelled in centimetres:

<draw>triangle ABC equilateral side:5 marks:on measures:cm

A right triangle, the right angle at A, legs 4 and 3, lengths in millimetres:

<draw>triangle ABC right sides:(4,3) marks:on measures:mm

A rhombus, a square and an equilateral triangle, each sharing a side with
the previous one. Only the first figure states its side: the others deduce
theirs from the shared edge, and each piece is flipped to the far side so
they abut instead of overlapping.

<draw>{
	rhombus ABEF side:4 angle:60
	square ABCD marks:on
	triangle BCG equilateral
	triangle ADK right:D DK:6 measures:cm
}

A triangle is also fixed by its three sides named one by one, AB:3 BC:4
CA:5 being exactly sides:(3,4,5) — the side AB first, then BC, then CA:

<draw>triangle ABC AB:3 BC:4 CA:5 measures:cm

A kite has two pairs of adjacent equal sides — the two upper sides equal,
the two lower sides equal, the apex angle between the upper pair. With
marks:on the two pairs are told apart by their ticks: a single tick on the
two upper sides, a double tick on the two lower ones, so the figure never
reads as a rhombus:

<draw>kite ABCD sides:(3,5) angle:80 marks:on measures:cm

A rectangle and a parallelogram share the same equality pattern — opposite
sides equal in pairs — so marks:on ticks one pair once and the other twice,
telling the two pairs apart. The rectangle adds the right-angle square at a
corner:

<draw>rectangle ABCD sides:(5,3) marks:on measures:cm

<draw>parallelogram ABCD sides:(5,3) angle:65 marks:on measures:cm

A trapezoid is given by its two parallel bases and its height. With the
default placement the legs come out equal — an isosceles trapezoid — so the
two bases are the unequal pair:

<draw>trapezoid ABCD bases:(6,3) height:3 measures:cm

The regular polygons read by name from the pentagon up; polygon with as
many points as wanted works the same way:

<draw>pentagon ABCDE side:3
<draw>hexagon ABCDEF side:2

A circle is given by its centre and a radius (or a diameter). The radius may
be a number, or a segment named by two placed points — radius:AB is the
compass opened to the span A–B:

<draw>circle O radius:3 measures:cm

Named after three already-placed points, the circle is their circumscribed
circle; add inscribed for the incircle of the triangle they form:

<draw>{
	triangle ABC sides:(4,5,6)
	circle ABC
}

<draw>{
	triangle ABC sides:(5,5,4)
	circle ABC inscribed
}

% =====================================================================
<h1>Point names, loops, and rotation
% =====================================================================

Point names are single letters written glued together — triangle ABC names
the points A, B and C. When a figure needs many points, or names with a
digit, they are written instead as a parenthesised, comma-separated list:
triangle (O, A0, B0). The rule is one of the first character: an opening
parenthesis introduces the list form, anything else is the glued form.

A draw block accepts the same loops as the rest of the language, written
for VARIABLE in FROM..TO { ... }. Inside a loop, ##k inserts the value of the
loop variable and ##{...} evaluates an expression, so point names and
measurements can be generated. The attribute rotate:θ turns a figure by θ
degrees about its first point.

Twelve isosceles triangles sharing an apex, each stepped 30° from the last,
fan out into a full turn. labels:off hides the vertex names, which would
otherwise crowd the centre:

<draw>{
	for k in 0..11 {
		triangle (O, A#k, B#k) isosceles side:4 base:2 rotate:#{k*30} labels:off
	}
}

A narrower base and a 45° step give an eight-pointed rosette:

<draw>{
	for k in 0..7 {
		triangle (O, A#k, B#k) isosceles side:5 base:1.5 rotate:#{k*45} labels:off
	}
}

% =====================================================================
<h1>Points and segments by coordinate
% =====================================================================

Below the named figures sit two low-level primitives. A point is placed at
explicit coordinates and a segment is drawn between two points. Coordinates
are declared once with let NAME = {x, y}, then referred to by name:

let P0 = {-1.6, -2.3}
let P1 = {-1, 1.5}
let P2 = {2.7, 3.1}

<draw>{
	point(P0)
	point(P1)
	point(P2)
	line(P1, P2)
}

An argument may also be a literal pair, and a segment takes measures: just
as a figure side does. Because each coordinate is an ordinary expression,
a loop can compute endpoints — twelve rays stepped 30° around a disc make a
sun:

<draw>{
	circle O radius:1.2
	for k in 0..11 {
		line({1.2*math.cos(k*math.pi/6), 1.2*math.sin(k*math.pi/6)}, {2.4*math.cos(k*math.pi/6), 2.4*math.sin(k*math.pi/6)})
	}
}

\end{document}