t-Formulae are used in integration to tackle rational expressions of tigonometric functions. After a spell in the cold, when they were not included in some A level specifications, they are now back in sixth form lessons.
It all starts with the subsitution,
from which the following functions can be derived,
, , .
These derivations can be made using compound trigonmetry fomulae. Alternatively, there is an engaging co-ordinate geometry derivation which has the merits of doubling up as an algebraic parametrisation of the circle.
Start with the following figure:
Euclid tells us that the angle subtended by the chord at the centre is twice the angle subtended at the circumference. The -axis providing a line of symmetry, gives the relationship between the angles and at and respectively.
Defining and creating a line, , through with gradient gives intersections with the -axis and the circle at and respectively.
The intersection point can then be found by solving the simulataneous equations:
, and, .
Substituting for leads to the quadratic,
which admits an easy factorisation once one acknowledges that it must have one root of .
giving the other root, and value for as .
Solving for gives the co-ordinates of . When viewed as two altenative parameterisations of the unit circle, the derivation of the formulae is complete.
There’s always space on the inter-web for another proof of Pythagoras’s Theorem. Here’s one that uses the following equal areas property of parallelograms.
This kind of area chopping and shape translation is a feature of Euclidean geometry and our senses support it’s veracity at the order of size of the classroom.
The squares on the sides of a right-angle triangle set up a system of parallel lines which can then be used to demonstrate the Theorem using the above equal areas property.
The thread does not stop here though. Taking the parallel line structure which makes this work we get a generalisation of Pythagoras to non-right-angled triangles with the area of the parallelogram on the longest side being the sum of the areas of those constructed on the other two sides.
In the geometry of mariners, where straight lines are Great Circles, the angle sum of a triangle is more than 180 degrees.
This method for finding the centre, and by extension the equation, of a circle given three non-colinear points, brings the ancient textbook master Euclid onto Descartes’ coordinate plane and right into the 21 century classroom.