## Geometry
+### Area of a triangle
+
+$A={1 \over 2} a b \sin C$
+
### Parallel lines
If parallel lines are crossed by transversal:
### Circle geometry
--  The angle at the centre of a circle is twice the angle at the circumference subtended by the arc
--  the angle in a semicircle is a right angle
--  angles in the same segment of a circle are equal
+- {#id .class width=40%} The angle at the centre of a circle is twice the angle at the circumference subtended by the arc
+- {#id .class width=40%} the angle in a semicircle is a right angle
+- {#id .class width=40%} angles in the same segment of a circle are equal
- ![]()
+
+
+## Ellipses and hyperbolas
+
+#### Ellipses
+
+$${(x-h)^2 \over a^2}+{(y-k)^2 \over b^2} = 1$$
+
+#### Hyperbolas
+
+$${(x-h)^2 \over a^2} - {(y-k)^2 \over b^2} = 1$$
+
+- centre at $(h,k)$
+- asymptotes at $y-k=\pm{b \over a}(x-h)$
+
+${(x-h)^2 \over a^2} - {(y-k)^2 \over b^2} = 1$ and ${(y-k)^2 \over b^2} - {(x-h)^2 \over a^2} = 1$ are **conjugate hyperbolas**
+
+## Modulus function
+
+$$|x|=\sqrt{x^2}$$
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