**Figure 1: Graph of $\cos(3A)=4\cos ^{3}(A)-3\cos(A)$**
This formula and/or graph are usually used to calculate $\cos(A)$ if $\cos(3A)$ is given.

The cosine triple angle formula is: $\cos(3\theta )=4\cos ^{3}\theta -3\cos \theta .$ This formula, of form $y=4x^{3}-3x$ , permits $\cos(3\theta )$ to be calculated if $\cos \theta$ is known.

If $\cos(3\theta )$ is known and the value of $\cos \theta$ is desired, this identity becomes: $4\cos ^{3}\theta -3\cos \theta -\cos(3\theta )=0.$ $\cos \theta$ is the solution of this cubic equation.

In fact this equation has three solutions, the other two being $\cos(\theta \pm 120^{\circ }).$

$\cos(3(\theta \pm 120^{\circ }))=\cos(3\theta \pm 360^{\circ })=\cos(3\theta ).$

Perhaps the simplest solution of the cubic equation with three real roots depends on the calculation of $\cos {\frac {A}{3}}.$ The two are mutually dependent.

This page contains a method for calculating $\cos \theta$ from $\cos(3\theta )$ that is not dependent on:

calculus,

the solution of a cubic equation, or

either value of $\theta$ or $3\theta .$

Background

The value ${\frac {1}{3}}$ can be approximated by the sequence ${\frac {1}{4}}+{\frac {1}{4^{2}}}+{\frac {1}{4^{3}}}+{\frac {1}{4^{4}}}+\cdots$

For example ${\frac {1}{4}}+{\frac {1}{4^{2}}}+{\frac {1}{4^{3}}}+\cdots +{\frac {1}{4^{20}}}$ equals $0.33333333333303017\cdots$

Testing with sequences of different lengths indicates that, if sequence contains $N$ terms, result is accurate to $0.6N$ places of decimals.

In this example the sequence contains $20$ terms and result is accurate to $12$ places of decimals.

$\cos {\frac {A}{3}}=\cos(A({\frac {1}{4}}+{\frac {1}{4^{2}}}+{\frac {1}{4^{3}}}+{\frac {1}{4^{4}}}+\cdots ))$ $=\cos({\frac {A}{4}}+{\frac {A}{4^{2}}}+{\frac {A}{4^{3}}}+{\frac {A}{4^{4}}}+\cdots )$

Implementation

# python code
from decimal import *
getcontext().prec = 100

def cosQuarterAngle (cos4A) :
    cos4A = Decimal(str(cos4A))
    cos2A = ((cos4A+1)/2).sqrt()
    return ((cos2A+1)/2).sqrt()

def cosAplusB (cosA,cosB) :
    cosA,cosB = [ Decimal(str(v)) for v in (cosA,cosB) ]
    sinA = (1 - cosA*cosA).sqrt()
    sinB = (1 - cosB*cosB).sqrt()
    v1 = cosA*cosB
    v2 = sinA*sinB
    return v1 - v2, v1 + v2

def cosAfrom_cos3A(cos3A) :
    cos3A = Decimal(str(cos3A))
    if 1 >= cos3A >= -1 : pass
    else :
        print ('cosAfrom_cos3A(cos3A) : cos3A not in valid range.')
        return None
    if cos3A == 0 :
        # 3A = 90 , return cos30
        # 3A = 90 + 360 = 450 , return cos150, 30+120
        # 3A = 90 + 720 = 810 , return cos270, 30-120 = -90
        cos30 = Decimal(3).sqrt()/2
        return cos30, -cos30, Decimal(0)
    cos60 = Decimal('0.5')
    if cos3A == 1 :
        # 3A = 0 , return cos0
        # 3A = 360 , return cos120, 0+120
        # 3A = 720 , return cos240, 0-120
        return Decimal(1), -cos60, -cos60
    if cos3A == -1 :
        # 3A = 180 , return cos60
        # 3A = 180 + 360 = 540 , return cos180, 60+120
        # 3A = 180 + 720 = 900 , return cos300, 60-120 = -60
        return cos60, Decimal(-1), cos60

    cosA = cosQuarterAngle (cos3A)
    next = cosQuarterAngle (cosA)
    count = 0
    while(next != 1):
        cosA = cosAplusB (cosA,next) [0]
        next = cosQuarterAngle (next)
        count += 1
    print ('count =',count)
    cos120 = -cos60
    return (cosA,) + cosAplusB (cosA,cos120)

Result is accurate to about half the precision. For example, if precision is set to $100,$ result is accurate to approximately $50$ places of decimals, and is achieved with about 81 passes through loop.

Cosine(A/3)

Background

Implementation