If you remember the above three formulas, you can derive pretty much all the other trigonometric identities.
Double angle formulas
Starting from the sico-sico identity as explained above, and setting a = b = x, we can derive the following identity:
sin(2x) = 2 sin(x) cos(x).
Starting from the coco-sisi identity, we obtain cos(2x) = cos2(x)− sin2(x)
= 2 cos2(x)− 1 = 2 1 − sin2(x) − 1 = 1 − 2 sin2(x).
The formulas for expressing sin(2x) and cos(2x) in terms of sin(x) and cos(x) are called double angle formulas.
If we rewrite the double-angle formula for cos(2x) to isolate the sin2or the cos2 term, we obtain the power-reduction formulas:
cos2(x) = 1
2(1 + cos(2x)) , sin2(x) = 1
2(1− cos(2x)) . Self similarity
Sin and cos are periodic functions with period 2π. Adding a multiple of 2π to the function’s input does not change the function:
sin(x + 2π) = sin(x + 124π) = sin(x), cos(x + 2π) = cos(x).
Furthermore, sin and cos are self similar within each 2π cycle:
sin(π− x) = sin(x), cos(π− x) = − cos(x).
Sin is cos, cos is sin
It shouldn’t be surprising if I tell you that sin and cos are actually
π
2-shifted versions of each other:
cos(x) = sin x+π
2
= sinπ 2−x
, sin(x) = cos x−π
2
= cosπ 2−x
.
1.13 GEOMETRY 65
The above formulas will come in handy when you need to find some unknown in an equation, or when you are trying to simplify a trigono-metric expression. I am not saying you should necessarily memorize them, but you should be aware that they exist.
1.13 Geometry
Triangles
The area of a triangle is equal to 12 times the length of its base times its height:
A = 1 2aha.
Note that ha is the height of the triangle relative to the side a.
The perimeter of a triangle is
P = a + b + c.
Consider a triangle with internal angles α, β and γ. The sum of the inner angles in any triangle is equal to two right angles: α + β + γ = 180◦.
Sine rule The sine law is a
sin(α) = b
sin(β) = c sin(γ),
where α is the angle opposite to a, β is the angle opposite to b, and γ is the angle opposite to c.
Cosine rule The cosine rules are
a2= b2+ c2− 2bc cos(α), b2= a2+ c2− 2ac cos(β), c2= a2+ b2− 2ab cos(γ).
Sphere
A sphere is described by the equation x2+ y2+ z2= r2. The surface area of a sphere is
A = 4πr2, and the volume of a sphere is
V = 4 3πr3.
Cylinder
The surface area of a cylinder consists of the top and bottom circular surfaces, plus the area of the side of the cylinder:
A = 2 πr2 + (2πr)h.
The formula for the volume of a cylinder is the product of the area of the cylinder’s base times its height:
V = πr2 h.
www.ebook3000.com
1.14 CIRCLE 67
Example You open the hood of your car and see 2.0 L written on top of the engine. The 2.0 L refers to the combined volume of the four pistons, which are cylindrical in shape. The owner’s manual tells you the diameter of each piston (bore) is 87.5 mm, and the height of each piston (stroke) is 83.1 mm. Verify that the total volume of the cylinder displacement of your engine is indeed 1998789 mm3≈ 2 L.
1.14 Circle
The circle is a set of points located a constant distance from a centre point. This geometrical shape appears in many situations.
Definitions
• r: the radius of the circle
• A: the area of the circle
• C: the circumference of the circle
• (x, y): a point on the circle
• θ: the angle (measured from the x-axis) of some point on the circle
Formulas
A circle with radius r centred at the origin is described by the equation x2+ y2= r2.
All points (x, y) that satisfy this equation are part of the circle.
Rather than staying centred at the ori-gin, the circle’s centre can be located at any point (p, q) on the plane:
(x− p)2+ (y− q)2= r2. Explicit function
The equation of a circle is a relation or an implicit function involving x and y. To ob-tain an explicit function y = f(x) for the circle, we can solve for y to obtain
y =p
r2− x2, −r ≤ x ≤ r, and
y =−p
r2− x2, −r ≤ x ≤ r.
The explicit expression is really two functions, because a vertical line crosses the circle in two places. The first function corresponds to the top half of the circle, and the second function corresponds to the bottom half.
Polar coordinates
Circles are so common in mathematics that mathematicians developed a special “circular coordinate system” in order to describe them more easily.
It is possible to specify the coordinates (x, y) of any point on the circle in terms of the polar coordinates r∠θ, where r measures the distance of the point from the origin, and θ is the angle measured from the x-axis.
To convert from the polar coordinates r∠θ to the (x, y) coordinates, use the trigonomet-ric functions cos and sin:
x = r cos θ and y = r sin θ.
Parametric equation
We can describe all the points on the circle if we specify a fixed radius r and vary the angle θ over all angles: θ ∈ [0, 360◦). A para-metric equation specifies the coordinates (x(θ), y(θ)) for the points on a curve, for all values of the parameter θ. The parametric equation for a circle of radius r is given by
{(x, y) ∈ R2 | x = r cos θ, y = r sin θ, θ ∈ [0, 360◦)}.
Try to visualize the curve traced by the point (x(θ), y(θ)) = (r cos θ, r sin θ) as θ varies from 0◦to 360◦. The point will trace out a circle of radius r. If we let the parameter θ vary over a smaller interval, we’ll obtain subsets of the circle. For example, the parametric equation for the top half of the circle is
{(x, y) ∈ R2| x = r cos θ, y = r sin θ, θ ∈ [0, 180◦]}.
The top half of the circle is also described by {(x, y) ∈ R2 | y =
√r2− x2, x∈ [−r, r]}, where the parameter used is the x-coordinate.
Area
The area of a circle of radius r is A = πr2.
1.15 SOLVING SYSTEMS OF LINEAR EQUATIONS 69
Circumference and arc length The circumference of a circle is
C = 2πr.
This is the total length you can measure by following the curve all the way around to trace the outline of the entire circle.
O r
`
θ = 57◦ What is the length of a part of the circle?
Say you have a piece of the circle, called an arc, and that piece corresponds to the angle θ = 57◦. What is the arc’s length `?
If the circle’s total length C = 2πr repre-sents a full 360◦ turn around the circle, then the arc length ` for a portion of the circle cor-responding to the angle θ is
` = 2πr θ 360.
The arc length ` depends on r, the angle θ, and a factor of 3602π.
Radians
Though degrees are commonly used as a measurement unit for angles, it’s much better to measure angles in radians, since radians are the natural units for measuring angles. The conversion ratio between degrees and radians is
2π[rad] = 360◦.
When measuring angles in radians, the arc length is given by:
` = rθrad.
Measuring angles in radians is equivalent to measuring arc length on a circle with radius r = 1.
1.15 Solving systems of linear equations
We know that solving equations with one unknown—like 2x+4 = 7x, for instance—requires manipulating both sides of the equation until the unknown variable is isolated on one side. For this instance, we can subtract 2x from both sides of the equation to obtain 4 = 5x, which simplifies to x = 45.
What about the case when you are given two equations and must solve for two unknowns? For example,
x + 2y = 5, 3x + 9y = 21.
Can you find values of x and y that satisfy both equations?
Concepts
• x, y: the two unknowns in the equations
• eq1, eq2: a system of two equations that must be solved simul-taneously. These equations will look like
a1x + b1y = c1, a2x + b2y = c2, where as, bs, and cs are given constants.
Principles
If you have n equations and n unknowns, you can solve the equations simultaneously and find the values of the unknowns. There are several different approaches for solving equations simultaneously. We’ll learn about three different approaches in this section.