Show that ##\sin(\arctan x) < x < \tan(\arcsin x)##

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SUMMARY

The discussion focuses on proving the inequality $$ \sin(\arctan x) < x < \tan(\arcsin x) $$ for the interval $$ 0 < x < 1 $$. The user establishes that $$ \sin(\arctan x) $$ can be expressed as $$ \frac{x}{\sqrt{x^2+1}} $$ and seeks assistance in deriving the expression for $$ \tan(\arcsin x) $$ to complete the proof. The community emphasizes the relationship between the functions involved and suggests comparing $$ \arctan(x) $$ and $$ \arcsin(x) $$ directly with $$ x $$ instead of deriving the tangent expression.

PREREQUISITES
  • Understanding of trigonometric functions: sine, tangent, and their inverses.
  • Familiarity with the properties of right triangles and the Pythagorean theorem.
  • Knowledge of inequalities involving trigonometric functions in specific intervals.
  • Basic algebraic manipulation skills to handle expressions involving square roots and fractions.
NEXT STEPS
  • Study the properties of trigonometric functions in the interval $$ 0 < x < 1 $$.
  • Research the relationship between $$ \arctan(x) $$ and $$ \arcsin(x) $$ for values in the specified range.
  • Learn how to derive expressions for $$ \tan(\arcsin(x)) $$ using trigonometric identities.
  • Explore proofs involving inequalities of trigonometric functions and their applications.
USEFUL FOR

Mathematics students, educators, and anyone interested in trigonometric inequalities and their proofs will benefit from this discussion.

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The problem
For ##0 < x < 1 ## . Show that
$$ \sin(\arctan x) < x < \tan(\arcsin x) $$

The attempt

I know that ## \sin x < x < \tan x ## is true for ## 0 < x < \ \pi / 2 ##

220px-Sinxoverx.png
x - is by definition the length DA (in radians)

I draw a right triangle with sides x and 1 and with hypotenuse ## \sqrt{x^2+1} ##

## \sin v = \frac{x}{\sqrt{x^2+1}} ##

## \tan v = \frac{x}{1} \Rightarrow v = \arctan \ x ##

## \sin (\arctan \ x) = \frac{x}{\sqrt{x^2+1}} ##I am trying to write an expression for ## \tan(\arcsin(x)) ##

## \sin v = \frac{x}{\sqrt{x^2+1}} \Rightarrow v = \arcsin \left( \frac{x}{\sqrt{x^2+1}} \right)##

## \tan v = x \Rightarrow \tan \left( \arcsin \left( \frac{x}{\sqrt{x^2+1}} \right) \right) = x##

But I fail. Please Help me write an expression for ## \tan(\arcsin(x)) ##.
 
Last edited:
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I don't think this is necessary. You can compare arctan(x) and arcsin(x) with x.
 

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