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Functions question

2010 · Shift 1 · Q52
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  5. /2010 · Shift 1 · Q52

Functions question

2010 · Shift 1 · Q52

JEE AdvancedMathematicsFunctionsMCQ+3 / −1
Let f,gf, gf,g and hhh be real valued functions defined on the interval [0,1][0,1][0,1] by f(x)=ex2+e−x2f(x)=e^{x^2}+e^{-x^2}f(x)=ex2+e−x2, g(x)=xex2+e−x2g(x)=x e^{x^2}+e^{-x^2}g(x)=xex2+e−x2 and h(x)=x2ex2+e−x2h(x)=x^2 e^{x^2}+e^{-x^2}h(x)=x2ex2+e−x2. If a,ba, ba,b and ccc denote, respectively, the absolute maximum of f,gf, gf,g and hhh on [0,1][0,1][0,1], then :
  1. A
    a=ba=ba=b and ceqbc eq bceqb
  2. B
    a=ca=ca=c and aeqba eq baeqb
  3. C
    aeqba eq baeqb and ceqbc eq bceqb
  4. D
    a=b=ca=b=ca=b=c
View written solutionFree

Correct answer: D

The user wants me to find the absolute maximum values of three functions, f(x)f(x)f(x), g(x)g(x)g(x), and h(x)h(x)h(x), on the interval [0,1][0, 1][0,1] and compare them.

The functions are:

  • f(x)=ex2+e−x2f(x) = e^{x^2} + e^{-x^2}f(x)=ex2+e−x2
  • g(x)=xex2+e−x2g(x) = x e^{x^2} + e^{-x^2}g(x)=xex2+e−x2
  • h(x)=x2ex2+e−x2h(x) = x^2 e^{x^2} + e^{-x^2}h(x)=x2ex2+e−x2

The absolute maximum of a continuous function on a closed interval occurs either at the endpoints of the interval or at critical points within the interval.

Step 1: Analyze the function f(x)

  1. Find the derivative of f(x)f(x)f(x): f′(x)=ddx(ex2+e−x2)=ex2(2x)+e−x2(−2x)=2x(ex2−e−x2)f'(x) = \frac{d}{dx} (e^{x^2} + e^{-x^2}) = e^{x^2}(2x) + e^{-x^2}(-2x) = 2x(e^{x^2} - e^{-x^2})f′(x)=dxd​(ex2+e−x2)=ex2(2x)+e−x2(−2x)=2x(ex2−e−x2)
  2. Find critical points by setting f′(x)=0f'(x)=0f′(x)=0: 2x(ex2−e−x2)=02x(e^{x^2} - e^{-x^2}) = 02x(ex2−e−x2)=0. This implies x=0x=0x=0 or ex2=e−x2e^{x^2} = e^{-x^2}ex2=e−x2, which means x2=−x2x^2 = -x^2x2=−x2, so x=0x=0x=0. The only critical point is at x=0x=0x=0, which is an endpoint.
  3. Determine the behavior of f(x)f(x)f(x) on [0,1][0,1][0,1]: For x∈(0,1]x \in (0, 1]x∈(0,1], we have x2>0x^2 > 0x2>0, so −x2<x2-x^2 < x^2−x2<x2. Since the exponential function ete^tet is strictly increasing, e−x2<ex2e^{-x^2} < e^{x^2}e−x2<ex2, which means ex2−e−x2>0e^{x^2} - e^{-x^2} > 0ex2−e−x2>0. Also, 2x>02x > 02x>0. Therefore, f′(x)>0f'(x) > 0f′(x)>0 for all x∈(0,1]x \in (0, 1]x∈(0,1].
  4. This means f(x)f(x)f(x) is a strictly increasing function on the interval [0,1][0, 1][0,1]. The absolute maximum must occur at the right endpoint, x=1x=1x=1.
  5. Calculate the maximum value, aaa: a=f(1)=e12+e−12=e+e−1=e+1ea = f(1) = e^{1^2} + e^{-1^2} = e + e^{-1} = e + \frac{1}{e}a=f(1)=e12+e−12=e+e−1=e+e1​

Step 2: Analyze the function h(x)

  1. Find the derivative of h(x)h(x)h(x): h′(x)=ddx(x2ex2+e−x2)=(2xex2+x2ex2(2x))+e−x2(−2x)=2xex2+2x3ex2−2xe−x2h'(x) = \frac{d}{dx} (x^2 e^{x^2} + e^{-x^2}) = (2x e^{x^2} + x^2 e^{x^2}(2x)) + e^{-x^2}(-2x) = 2x e^{x^2} + 2x^3 e^{x^2} - 2x e^{-x^2}h′(x)=dxd​(x2ex2+e−x2)=(2xex2+x2ex2(2x))+e−x2(−2x)=2xex2+2x3ex2−2xe−x2 h′(x)=2x(ex2(1+x2)−e−x2)h'(x) = 2x(e^{x^2}(1+x^2) - e^{-x^2})h′(x)=2x(ex2(1+x2)−e−x2)
  2. Find critical points by setting h′(x)=0h'(x)=0h′(x)=0: One solution is x=0x=0x=0. For other solutions, we need ex2(1+x2)−e−x2=0e^{x^2}(1+x^2) - e^{-x^2} = 0ex2(1+x2)−e−x2=0, which is equivalent to (1+x2)=e−2x2(1+x^2) = e^{-2x^2}(1+x2)=e−2x2. Let k(x)=(1+x2)−e−2x2k(x) = (1+x^2) - e^{-2x^2}k(x)=(1+x2)−e−2x2. We see that k(0)=1−1=0k(0) = 1-1=0k(0)=1−1=0. Let's check the derivative of k(x)k(x)k(x): k′(x)=2x−e−2x2(−4x)=2x(1+2e−2x2)k'(x) = 2x - e^{-2x^2}(-4x) = 2x(1 + 2e^{-2x^2})k′(x)=2x−e−2x2(−4x)=2x(1+2e−2x2). For x∈(0,1]x \in (0, 1]x∈(0,1], k′(x)>0k'(x) > 0k′(x)>0. So k(x)k(x)k(x) is strictly increasing on [0,1][0,1][0,1]. Since k(0)=0k(0)=0k(0)=0, k(x)>0k(x)>0k(x)>0 for all x∈(0,1]x \in (0,1]x∈(0,1].
  3. This means the term (ex2(1+x2)−e−x2)(e^{x^2}(1+x^2) - e^{-x^2})(ex2(1+x2)−e−x2) is positive for x∈(0,1]x \in (0,1]x∈(0,1]. Therefore, h′(x)>0h'(x) > 0h′(x)>0 for all x∈(0,1]x \in (0,1]x∈(0,1].
  4. This means h(x)h(x)h(x) is also a strictly increasing function on the interval [0,1][0, 1][0,1]. The absolute maximum must occur at the right endpoint, x=1x=1x=1.
  5. Calculate the maximum value, ccc: c=h(1)=12e12+e−12=e+e−1=e+1ec = h(1) = 1^2 e^{1^2} + e^{-1^2} = e + e^{-1} = e + \frac{1}{e}c=h(1)=12e12+e−12=e+e−1=e+e1​

Step 3: Analyze the function g(x) and compare the functions

Instead of analyzing the derivative of g(x)g(x)g(x), which is more complex, we can compare the functions directly.

  1. Compare f(x)f(x)f(x) and g(x)g(x)g(x): f(x)−g(x)=(ex2+e−x2)−(xex2+e−x2)=ex2−xex2=(1−x)ex2f(x) - g(x) = (e^{x^2} + e^{-x^2}) - (x e^{x^2} + e^{-x^2}) = e^{x^2} - x e^{x^2} = (1-x)e^{x^2}f(x)−g(x)=(ex2+e−x2)−(xex2+e−x2)=ex2−xex2=(1−x)ex2. For x∈[0,1]x \in [0, 1]x∈[0,1], (1−x)≥0(1-x) \ge 0(1−x)≥0 and ex2>0e^{x^2} > 0ex2>0. So, f(x)−g(x)≥0f(x) - g(x) \ge 0f(x)−g(x)≥0, which means f(x)≥g(x)f(x) \ge g(x)f(x)≥g(x) for all x∈[0,1]x \in [0, 1]x∈[0,1].

  2. Compare g(x)g(x)g(x) and h(x)h(x)h(x): g(x)−h(x)=(xex2+e−x2)−(x2ex2+e−x2)=xex2−x2ex2=x(1−x)ex2g(x) - h(x) = (x e^{x^2} + e^{-x^2}) - (x^2 e^{x^2} + e^{-x^2}) = x e^{x^2} - x^2 e^{x^2} = x(1-x)e^{x^2}g(x)−h(x)=(xex2+e−x2)−(x2ex2+e−x2)=xex2−x2ex2=x(1−x)ex2. For x∈[0,1]x \in [0, 1]x∈[0,1], x≥0x \ge 0x≥0 and (1−x)≥0(1-x) \ge 0(1−x)≥0, and ex2>0e^{x^2} > 0ex2>0. So, g(x)−h(x)≥0g(x) - h(x) \ge 0g(x)−h(x)≥0, which means g(x)≥h(x)g(x) \ge h(x)g(x)≥h(x) for all x∈[0,1]x \in [0, 1]x∈[0,1].

  3. Combining these results, we have the inequality: f(x)≥g(x)≥h(x)f(x) \ge g(x) \ge h(x)f(x)≥g(x)≥h(x) for all x∈[0,1]x \in [0, 1]x∈[0,1].

Step 4: Determine the relationship between a, b, and c

Let a,b,ca, b, ca,b,c be the absolute maximum values of f,g,hf, g, hf,g,h respectively. From Step 1, a=max⁡x∈[0,1]f(x)=f(1)=e+1ea = \max_{x \in [0,1]} f(x) = f(1) = e + \frac{1}{e}a=maxx∈[0,1]​f(x)=f(1)=e+e1​. From Step 2, c=max⁡x∈[0,1]h(x)=h(1)=e+1ec = \max_{x \in [0,1]} h(x) = h(1) = e + \frac{1}{e}c=maxx∈[0,1]​h(x)=h(1)=e+e1​. So, we already know a=ca=ca=c.

Now let's find b=max⁡x∈[0,1]g(x)b = \max_{x \in [0,1]} g(x)b=maxx∈[0,1]​g(x). From the inequality f(x)≥g(x)f(x) \ge g(x)f(x)≥g(x), we can say that the maximum of g(x)g(x)g(x) cannot be greater than the maximum of f(x)f(x)f(x). So, b≤ab \le ab≤a. Thus, b≤e+1eb \le e + \frac{1}{e}b≤e+e1​.

The maximum value of a function must be at least its value at any point in the domain. So, bbb must be greater than or equal to g(1)g(1)g(1). g(1)=1⋅e12+e−12=e+1eg(1) = 1 \cdot e^{1^2} + e^{-1^2} = e + \frac{1}{e}g(1)=1⋅e12+e−12=e+e1​. So, b≥e+1eb \ge e + \frac{1}{e}b≥e+e1​.

Combining the two inequalities for bbb, we have b≤e+1eb \le e + \frac{1}{e}b≤e+e1​ and b≥e+1eb \ge e + \frac{1}{e}b≥e+e1​. This implies b=e+1eb = e + \frac{1}{e}b=e+e1​.

Therefore, we have a=b=c=e+1ea = b = c = e + \frac{1}{e}a=b=c=e+e1​.

Conclusion: The absolute maximums are a=e+1/ea = e + 1/ea=e+1/e, b=e+1/eb = e + 1/eb=e+1/e, and c=e+1/ec = e + 1/ec=e+1/e. Hence, a=b=ca=b=ca=b=c. This corresponds to option D.

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