Trigonometric sequence - Find Un out of Un+1 - hard

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SUMMARY

The discussion focuses on the trigonometric sequence defined by Un, where U0 = 2cos(x) and Un+1 = √(2 + Un). The user successfully calculates U1, U2, and U3 in terms of x, ultimately demonstrating that Un = 2cos(x / 2^n). The key simplification involves recognizing the relationship between cosine functions, specifically using the identity cos(2x) = cos²(x) - sin²(x) to derive U1 = 2cos(0.5x). This realization corrects earlier mistakes in simplification.

PREREQUISITES
  • Understanding of trigonometric identities, specifically cos(2x).
  • Familiarity with sequences and recursive definitions.
  • Basic algebraic manipulation skills.
  • Knowledge of square root functions and their properties.
NEXT STEPS
  • Study the derivation of trigonometric identities, focusing on double angle formulas.
  • Explore recursive sequences and their convergence properties.
  • Practice simplifying expressions involving square roots and trigonometric functions.
  • Learn about the properties of cosine functions and their applications in sequences.
USEFUL FOR

Students studying trigonometry, mathematicians working with sequences, and educators teaching advanced algebra concepts.

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Homework Statement



Un a sequence

0 < x < pi/2

U0 = 2cosx

Un+1 = quroot( 2 + Un)

1) Calculate U1, U2, and U3 in function of x (simplify maximum)

2) Show that Un = 2cos( x / 2^n)

Homework Equations


The Attempt at a Solution



1)
U1 = sqroot( 2 + 2cosx) = sqroot( 2 (1 + cosx))
= ?? (now , how do i simplify)

because , i should have when i have simplified a maximum :

U1 = 2cos(x/2)
= 2 (2cosx² - 1)
= 4cosx² - 2

So how do i go from sqroot( 2 + 2cosx) to 4cosx² - 2 ??
 
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Use \cos 2x = \cos^2 x - \sin^2 x
 
oh ...

i realized i did a mistake here :

this is wrong :
"U1 = 2cos(x/2)
= 2 (2cosx² - 1)
= 4cosx² - 2"

i can't believe , i confused cos(0.5x) and cos(2x)

////

now that i realized i did it like the other way round :

U1 = root( 2 + 2cos(x))
= root(2 + 2(2cos(0.5x)² - 1))
= root(4cos(0.5x)²)
= 2cos(0.5x)

Ok , managed to to do all the rest now , thanks
 
Last edited:

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