Inequality involving area under a curve

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SUMMARY

The forum discussion centers on proving the inequality $\displaystyle \int_0^1 \sqrt{1+(\cos y)^2} dy>\sqrt{x^2+(\sin x)^2}$ for every $x\in (0,\,1)$. Participants analyze the integral on the left side, which represents the area under the curve of the function $\sqrt{1+(\cos y)^2}$ over the interval from 0 to 1. The right side involves the expression $\sqrt{x^2+(\sin x)^2}$, which is evaluated for values of $x$ within the specified range. The conclusion drawn is that the integral consistently exceeds the right-hand expression, confirming the inequality.

PREREQUISITES
  • Understanding of definite integrals and area under curves
  • Familiarity with trigonometric functions, specifically sine and cosine
  • Knowledge of inequalities in calculus
  • Basic skills in mathematical proof techniques
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  • Explore advanced techniques in proving inequalities in calculus
  • Learn about the behavior of trigonometric functions within specified intervals
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Mathematicians, calculus students, and educators looking to deepen their understanding of inequalities involving integrals and trigonometric functions.

anemone
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Prove that for every $x\in (0,\,1)$ the following inequality holds:

$\displaystyle \int_0^1 \sqrt{1+(\cos y)^2} dy>\sqrt{x^2+(\sin x)^2}$
 
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Clearly $\displaystyle \int_0^1 \sqrt{1+(\cos y)^2} dy \ge \int_0^x \sqrt{1+(\cos y)^2} dy$ for each fixed $x\in (0,\,1)$. Observe that $\displaystyle \int_0^x \sqrt{1+(\cos y)^2} dy$ is the arc length of the function $f(y)=\sin y$ on the interval $[0,\,x]$ which is clearly strictly greater than the length of the straight line between the points $(0,\,0)$ and $(x,\, \sin x)$ which in turn is equal to $\sqrt{x^2+(\sin x)^2}$.
 

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