Problem 154 - A Complete Resource Book in Mathematics for JEE Main

Question

Assertion: \(\sin (\tan x) \geq x, \forall x \in\left[0, \frac{\pi}{4}\right]\) Reason: \(1-\cos x \leq \frac{x^{2}}{2}\)

Step-by-Step Solution

Answer

The reason does not support the assertion; it's unrelated.

Step 1: Verify the Assertion Statement

We need to check whether the inequality \( \sin(\tan x) \geq x \) holds for \( x \in \left[0, \frac{\pi}{4}\right] \). Evaluate \( \tan x \) within this range and ensure the outputted value for \( \sin(\tan x) \geq x \). Include critical points like \( x=0 \) and \( x=\frac{\pi}{4} \).

Step 2: Analyze the Reason Statement

The reason states that \( 1 - \cos x \leq \frac{x^2}{2} \). This can be derived from the Taylor expansion of \( \cos x \), where \( \cos x \approx 1 - \frac{x^2}{2} + \frac{x^4}{24} - \cdots \). Therefore, \( 1 - \cos x \approx \frac{x^2}{2} - \frac{x^4}{24} + \cdots \), confirming that \( 1 - \cos x \leq \frac{x^2}{2} \).

Step 3: Relate Assertion and Reason

Attempt to relate the assertion with the reason to understand whether the reason supports the assertion. Nevertheless, the reason addresses a different trigonometric inequality, without a direct link to \( \sin(\tan x) \geq x \).

Step 4: Conclusion

Given that the reason does not directly imply or support the assertion about \( \sin(\tan x) \geq x \), it's likely that the reason is correct but does not justify the assertion. The assertion must be verified independently.

Key Concepts

Taylor Expansion

Taylor expansion is a mathematical concept where you approximate a function as an infinite sum of terms calculated from the values of its derivatives at a single point. This is particularly useful in approximating functions that are otherwise difficult to work with. For example, the cosine function can be approximated using a Taylor series as:

• \( \cos x \approx 1 - \frac{x^2}{2} + \frac{x^4}{24} - \cdots \)

This series shows that the cosine of a small angle can be closely approximated by just a couple of terms. The Taylor expansion helps simplify trigonometric inequalities by offering an easy way to express functions in terms of powers of \( x \). In this particular problem, the reason statement uses Taylor expansion to prove that

• \( 1 - \cos x \leq \frac{x^2}{2} \)

This inequality is derived by removing higher order terms from the Taylor series to achieve a more manageable expression. It is important to note that Taylor expansions are more accurate for values of \( x \) closer to the point of expansion. Thus, they are commonly used for small values of \( x \), ensuring simpler forms without losing much precision.

Sine Function

The sine function is a fundamental element of trigonometry, describing the ratio of the length of the opposite side to the hypotenuse in a right triangle. For a given angle \( x \),

• \( \sin x \)

changes value between -1 and 1. It plays an essential role in various mathematical problems involving wave patterns and oscillatory behaviors. In the context of this exercise, the sine of the tangent function was studied. The inequality \(\sin(\tan x) \geq x\)is under focus, particularly in the interval \([0, \frac{\pi}{4}]\). Evaluating this inequality involves understanding the behavior of sine and tangent functions together. Remember, within this specific range, both sine and tangent functions are increasing, and their values are positive.
Moreover, the sine function’s behavior near small angles (such as those between 0 and \(\frac{\pi}{4}\)) helps to simplify complex inequalities due to its predictable increase.

Cosine Function

The cosine function, another cornerstone of trigonometry, is defined as the adjacent side over the hypotenuse in a right triangle. Like the sine function, the cosine function measures how an angle affects the rotation in a circular path. Cosine values range from -1 to 1, thus allowing significant explorations in trigonometric identities and inequalities.
The exercise talks about an inequality

• \( 1 - \cos x \leq \frac{x^2}{2} \)

which indicates the difference between 1 and the cosine of \( x \). This inequality is derived from the Taylor series expansion for small \( x \). For small angles,

• \( \cos x \approx 1 - \frac{x^2}{2} \)

The approximation becomes tighter with smaller values of \( x \). This is crucial for simplifying expressions and developing easier interpretations of trigonometric inequalities. In trigonometric inequalities, understanding the cosine function often allows the simplification of expressions by using known approximations for small values of \( x \).