Functions

JEE Mathematics · 125 questions · Page 9 of 13 · Click an option or "Show Solution" to reveal answer

Q81
Let f(x)=2xn+λ,λR,nNf(x) = 2{x^n} + \lambda ,\lambda \in R,n \in N, and f(4)=133,f(5)=255f(4) = 133,f(5) = 255. Then the sum of all the positive integer divisors of (f(3)f(2))(f(3) - f(2)) is
A 60
B 58
C 61
D 59
Correct Answer
Option A
Solution

f(x)=2xn+λ,λR,nNf(x)=2 x^{n}+\lambda, \lambda \in \mathbb{R}, n \in \mathbb{N} f(4)=24n+λ=133,f(5)=25n+λ=255f(4)=2 \cdot 4^{n}+\lambda=133, f(5)=2 \cdot 5^{n}+\lambda=255 f(5)f(4)=2(5n4n)=122n=3f(5)-f(4)=2 \cdot\left(5^{n} \cdot 4^{n}\right)=122 \Rightarrow n=3 f(3)f(2)=2(3n2n)=2(3323)=2×19\Rightarrow f(3)-f(2)=2 \cdot\left(3^{n} \cdot 2^{n}\right)=2 \cdot\left(3^{3}-2^{3}\right)=2 \times 19 Required sum =1+2+19+38=60=1+2+19+38=60

Q82
Let f(x)f(x) be a function such that f(x+y)=f(x).f(y)f(x+y)=f(x).f(y) for all x,yNx,y\in \mathbb{N}. If f(1)=3f(1)=3 and k=1nf(k)=3279\sum\limits_{k = 1}^n {f(k) = 3279} , then the value of n is
A 9
B 7
C 6
D 8
Correct Answer
Option B
Solution
f(x+y)=f(x)f(y)x,yN,f(1)=3f(2)=f2(1)=32f(3)=f(1)f(2)=33f(4)=34f(k)=3kk=1nf(k)=3279f(1)+f(2)+f(3)++f(k)=32793+32+33+3k=32793(3k1)31=32793k12=10933k1=21863k=2187So, k=7\begin{aligned} & \mathrm{f}(\mathrm{x}+\mathrm{y})=\mathrm{f}(\mathrm{x}) \cdot \mathrm{f}(\mathrm{y}) \forall \mathrm{x}, \mathrm{y} \in \mathrm{N}, \mathrm{f}(1)=3 \\\\ & \mathrm{f}(2)=\mathrm{f}^2(1)=3^2 \\\\ & \mathrm{f}(3)=\mathrm{f}(1) \mathrm{f}(2)=3^3 \\\\ & \mathrm{f}(4)=3^4 \\\\ & \mathrm{f}(\mathrm{k})=3^{\mathrm{k}} \\\\ & \sum_{\mathrm{k}=1}^{\mathrm{n}} \mathrm{f}(\mathrm{k})=3279 \\\\ & \mathrm{f}(1)+\mathrm{f}(2)+\mathrm{f}(3)+\ldots \ldots \ldots+\mathrm{f}(\mathrm{k})=3279 \\\\ & 3+3^2+3^3+\ldots \ldots \ldots 3^{\mathrm{k}}=3279 \\\\ & \frac{3\left(3^{\mathrm{k}}-1\right)}{3-1}=3279 \\\\ & \frac{3^{\mathrm{k}}-1}{2}=1093 \\\\ & 3^{\mathrm{k}}-1=2186 \\\\ & 3^{\mathrm{k}}=2187 \\\\ & \text{So, } \mathrm{k}=7 \end{aligned}
Q83
For xRx \in \mathbb{R}, two real valued functions f(x)f(x) and g(x)g(x) are such that, g(x)=x+1g(x)=\sqrt{x}+1 and fg(x)=x+3xf \circ g(x)=x+3-\sqrt{x}. Then f(0)f(0) is equal to
A 5
B 0
C -3
D 1
Correct Answer
Option A
Solution
g(x)=x+1fog(x)=x+3x=(x+1)23(x+1)+5=g2(x)3g(x)+5f(x)=x23x+5f(0)=5\begin{aligned} & g(x)=\sqrt{x}+1 \\\\ & \operatorname{fog}(x)=x+3-\sqrt{x} \\\\ & =(\sqrt{x}+1)^2-3(\sqrt{x}+1)+5 \\\\ & =g^2(x)-3 g(x)+5 \\\\ & \Rightarrow f(x)=x^2-3 x+5 \\\\ & \therefore f(0)=5 \end{aligned}

But, if we consider the domain of the composite function fg(x)f \circ g(x) then in that case f(0)f(0) will be not defined as g(x)\mathrm{g}(\mathrm{x}) cannot be equal to zero.

Q84
Let D\mathrm{D} be the domain of the function f(x)=sin1(log3x(6+2log3x5x))f(x)=\sin ^{-1}\left(\log _{3 x}\left(\dfrac{6+2 \log _{3} x}{-5 x}\right)\right). If the range of the function g:DR\mathrm{g}: \mathrm{D} \rightarrow \mathbb{R} defined by g(x)=x[x],([x]\mathrm{g}(x)=x-[x],([x] is the greatest integer function), is (α,β)(\alpha, \beta), then α2+5β\alpha^{2}+\dfrac{5}{\beta} is equal to
A 45
B 136
C 46
D nearly 135
Correct Answer
Option D
Solution

First, the function f(x)=sin1(log3x(6+2log3x5x))f(x) = \sin^{-1}(\log_{3x}(\dfrac{6 + 2 \log_3{x}}{-5x})) has several restrictions : Since the arcsine function sin1(x)\sin^{-1}(x) is only defined for 1x1-1\leq x\leq 1, this means that log3x(6+2log3x5x)\log_{3x}(\dfrac{6 + 2 \log _3 x}{-5 x}) must be between -1 and 1.

For the logarithm to be defined, 3x>0x>03x > 0 \Rightarrow x > 0 .......(1) because the base of a logarithm must be greater than 0 and not equal to 1.

Also, x13x \neq \dfrac{1}{3} .......(2) as the base cannot be 1.

Moreover, the inner function of the logarithm 6+2log3x5x\dfrac{6 + 2 \log_3{x}}{-5x} must be greater than 0. \Rightarrow 6+2log3x0)6+2 \log _3 x0) $ \begin{aligned} \Rightarrow & \log _3 x The next step is to solve the inequality for -1 \leq \log_{3x}(\frac{6+2 \log _3 x}{-5 x}) \leq 1.Todothis,wemaketheobservationthatforthelogarithmicparttobewithin. To do this, we make the observation that for the logarithmic part to be within [-1,1],itmustbetruethat, it must be true that 3x \leq \frac{6+2 \log_3 x}{-5x} \leq \frac{1}{3x}.Solvingtheinequality. Solving the inequality 15x^2 + 6 + 2 \log_3 x \geq 0,wefindthat, we find that x \in(0, \frac{1}{27})........(4)Likewise,solvingtheinequality ........(4) Likewise, solving the inequality 6+2 \log_3 x + \frac{5}{3} \geq 0,wefindthat, we find that x \geq 3^{-\frac{23}{6}}........(5)CombiningEquations(3),(4),and(5),theintersectionofalltheseintervalsis ........(5) Combining Equations (3), (4), and (5), the intersection of all these intervals is x \in[3^{-\frac{23}{6}}, \frac{1}{27}).Now,considerthefunction. Now, consider the function g(x) = x - [x],where, where [x]isthegreatestintegerfunction.Forthisfunction,therangeisthefractionalpartof is the greatest integer function. For this function, the range is the fractional part of x.Inthiscase,therange. In this case, the range (\alpha, \beta)isgivenbytheminimumandmaximumpossiblevaluesof is given by the minimum and maximum possible values of xinitsdomain.Hence, in its domain. Hence, \alpha = 3^{-\frac{23}{6}}and and \beta = \frac{1}{27}.Finally,substitutethesevaluesintotheequation. Finally, substitute these values into the equation \alpha^{2}+\frac{5}{\beta}:: \alpha^{2}+\frac{5}{\beta} = (3^{-\frac{23}{6}})^2 + \frac{5}{\frac{1}{27}} = 3^{-\frac{23}{3}} + 135=135.0002198.Since = 135.0002198. Since 3^{-\frac{23}{3}}=0.0002198isanextremelysmallnumber,itsapproximately0.So, = 0.0002198 is an extremely small number, it's approximately 0. So, \alpha^{2}+\frac{5}{\beta} \approx 135$.

Q85
The domain of the function f(x)=1[x]23[x]10f(x)=\dfrac{1}{\sqrt{[x]^{2}-3[x]-10}} is : ( where [x][\mathrm{x}] denotes the greatest integer less than or equal to xx )
A (,2)[6,)(-\infty,-2) \cup[6, \infty)
B (,3][6,)(-\infty,-3] \cup[6, \infty)
C (,2)(5,)(-\infty,-2) \cup(5, \infty)
D (,3](5,)(-\infty,-3] \cup(5, \infty)
Correct Answer
Option A
Solution
f(x)=1[x]23[x]10f(x)=\frac{1}{\sqrt{[x]^2-3[x]-10}}

For Domain [x]23[x]10>0[x]^2-3[x]-10>0

([x]5)([x]+2)>0[x](,2)(5,)x(,2)[6,)\begin{aligned} & \Rightarrow ([x]-5)([x]+2)>0 \\\\ & \Rightarrow [x] \in(-\infty,-2) \cup(5, \infty) \\\\ & \therefore x \in(-\infty,-2) \cup[6, \infty) \end{aligned}
Q86
If f(x)=(tan1)x+loge(123)xloge(1234)(tan1),x>0f(x) = {{(\tan 1^\circ )x + {{\log }_e}(123)} \over {x{{\log }_e}(1234) - (\tan 1^\circ )}},x > 0, then the least value of f(f(x))+f(f(4x))f(f(x)) + f\left( {f\left( {{4 \over x}} \right)} \right) is :
A 2
B 4
C 0
D 8
Correct Answer
Option B
Solution

Given that f(x)=(tan1)x+loge(123)xloge(1234)(tan1)f(x)=\dfrac{\left(\tan 1^{\circ}\right) x+\log _e(123)}{x \log _e(1234)-\left(\tan 1^{\circ}\right)} Let us consider a similar function of (x)(x), f(x)=Ax+BCxA\therefore f(x)=\dfrac{A x+B}{C x-A}  Now, \text{ Now, }

f(f(x))=A(Ax+BCxA)+BC(Ax+BCxA)A=A2x+AB+BCxABACx+BCACx+A2=x(A2+BC)(BC+A2)=x\begin{aligned} &f(f(x)) =\frac{A\left(\frac{A x+B}{C x-A}\right)+B}{C\left(\frac{A x+B}{C x-A}\right)-A} \\\\ & =\frac{A^2 x+A B+B C x-A B}{A C x+B C-A C x+A^2} \\\\ & =\frac{x\left(A^2+B C\right)}{\left(B C+A^2\right)}=x \end{aligned}
f(f(x))=x Similarly, f(f(4x))=4x Apply, AM  GM (x+4x)2x4x=4(x>0)f(f(x))+f(f(4x))4\begin{aligned} & \therefore \quad f(f(x))=x \\\\ & \text{ Similarly, } f\left(f\left(\frac{4}{x}\right)\right)=\frac{4}{x} \\\\ & \text{ Apply, AM } \geq \text{ GM } \\\\ & \left(x+\frac{4}{x}\right) \geq 2 \sqrt{x \cdot \frac{4}{x}}=4(x>0) \\\\ & \Rightarrow f(f(x))+f\left(f\left(\frac{4}{x}\right)\right) \geq 4 \end{aligned}

Hence, the least value is 4 .

Q87
Let f,g:RRf, g: \mathbf{R} \rightarrow \mathbf{R} be defined as : f(x)=x1 and g(x)={ex,x0x+1,x0.f(x)=|x-1| \text{ and } g(x)= \begin{cases}\mathrm{e}^x, & x \geq 0 \\ x+1, & x \leq 0 .\end{cases} Then the function f(g(x))f(g(x)) is
A neither one-one nor onto.
B one-one but not onto.
C both one-one and onto.
D onto but not one-one.
Correct Answer
Option A
Solution
f(x)={x1,x11xx<0g(x)={ex;x0x+1;x0\begin{aligned} & f(x)= \begin{cases}x-1, & x \geq 1 \\ 1-x & x<0\end{cases} \\ & g(x)=\left\{\begin{array}{cc} e^x & ; \quad x \geq 0 \\ x+1 & ; \quad x \leq 0 \end{array}\right. \end{aligned}
f(g(x))={g(x)1,g(x)11g(x)g(x)<1={ex1x0xx<0\begin{aligned} f(g(x)) & = \begin{cases}g(x)-1, & g(x) \geq 1 \\ 1-g(x) & g(x)<1\end{cases} \\ & =\left\{\begin{array}{cl} e^x-1 & x \geq 0 \\ -x & x<0 \end{array}\right. \end{aligned}
Q88
Let the sets A and B denote the domain and range respectively of the function f(x)=1xxf(x)=\dfrac{1}{\sqrt{\lceil x\rceil-x}}, where x\lceil x\rceil denotes the smallest integer greater than or equal to xx. Then among the statements (S1) : AB=(1,)NA \cap B=(1, \infty)-\mathbb{N} and (S2) : AB=(1,)A \cup B=(1, \infty)
A only (S2)(\mathrm{S} 2) is true
B only (S1) is true
C neither (S1) nor (S2) is true
D both (S1) and (S2) are true
Correct Answer
Option B
Solution
f(x)=1xxf(x)=\frac{1}{\sqrt{\lceil x\rceil-x}}

If xI,x=[x]\mathrm{x} \in \mathrm{I},\lceil\mathrm{x}\rceil=[\mathrm{x}] (greatest integer function) If xI,x=[x]+1x \notin I,\lceil x\rceil=[x]+1

f(x)={1[x]x,xI1[x]+1x,xIf(x)={1{x},xI, (does not exist) 11{x},xI domain of f(x)=RI\begin{aligned} & \Rightarrow \mathrm{f}(\mathrm{x})=\left\{\begin{array}{l} \frac{1}{\sqrt{[\mathrm{x}]-\mathrm{x}}}, \mathrm{x} \in \mathrm{I} \\\\ \frac{1}{\sqrt{[\mathrm{x}]+1-\mathrm{x}}}, \mathrm{x} \notin \mathrm{I} \end{array}\right. \\\\ & \Rightarrow \mathrm{f}(\mathrm{x})=\left\{\begin{array}{l} \frac{1}{\sqrt{-\{\mathrm{x}\}}}, \mathrm{x} \in \mathrm{I}, \text{ (does not exist) } \\\\ \frac{1}{\sqrt{1-\{\mathrm{x}\}}}, \mathrm{x} \notin \mathrm{I} \end{array}\right. \\\\ & \Rightarrow \text{ domain of } \mathrm{f}(\mathrm{x})=\mathrm{R}-\mathrm{I} \end{aligned}
 Now, f(x)=11{x},xI01 Range =(1,)A=RI\begin{aligned} & \text{ Now, } \mathrm{f}(\mathrm{x})=\frac{1}{\sqrt{1-\{\mathrm{x}\}}}, \mathrm{x} \notin \mathrm{I} \\\\ & \Rightarrow 01 \\\\ & \Rightarrow \text{ Range }=(1, \infty) \\\\ & \Rightarrow \mathrm{A}=\mathrm{R}-\mathrm{I} \end{aligned}
B=(1,) So, AB=(1,)NAB(1,)S1 is only correct \begin{aligned} & B=(1, \infty) \\\\ & \text{ So, } A \cap B=(1, \infty)-N \\\\ & A \cup B \neq(1, \infty) \\\\ & \Rightarrow S 1 \text{ is only correct } \end{aligned}
Q89
If the domain of the function f(x)=loge(2x+34x2+x3)+cos1(2x1x+2)f(x)=\log _e\left(\dfrac{2 x+3}{4 x^2+x-3}\right)+\cos ^{-1}\left(\dfrac{2 x-1}{x+2}\right) is (α,β](\alpha, \beta], then the value of 5β4α5 \beta-4 \alpha is equal to
A 9
B 12
C 11
D 10
Correct Answer
Option B
Solution
2x+34x2+x3>0 and 12x1x+212x+3(4x3)(x+1)>03x+1x+20&x3x+20\begin{aligned} & \frac{2 x+3}{4 x^2+x-3}>0 \text{ and }-1 \leq \frac{2 x-1}{x+2} \leq 1 \\ & \frac{2 x+3}{(4 x-3)(x+1)}>0 \quad \frac{3 x+1}{x+2} \geq 0 \& \frac{x-3}{x+2} \leq 0 \end{aligned}
(,2)[13,)(-\infty,-2) \cup\left[\frac{-1}{3}, \infty\right)

..... (1)

(2,3](-2,3]

..... (2)

[13,3]\left[\frac{-1}{3}, 3\right]

..... (3)

(1)(2)(3)\quad (1) \cap(2) \cap(3)
(34,3]α=34β=35β4α=153=12\begin{aligned} & \left(\frac{3}{4}, 3\right] \\ & \alpha=\frac{3}{4} \beta=3 \\ & 5 \beta-4 \alpha=15-3=12 \end{aligned}
Q90
If the domain of the function f(x)=x225(4x2)+log10(x2+2x15)f(x)=\dfrac{\sqrt{x^2-25}}{\left(4-x^2\right)}+\log _{10}\left(x^2+2 x-15\right) is (,α)[β,)(-\infty, \alpha) \cup[\beta, \infty), then α2+β3\alpha^2+\beta^3 is equal to :
A 140
B 175
C 125
D 150
Correct Answer
Option D
Solution

To find the domain of the function

f(x)=x225(4x2)+log10(x2+2x15),f(x) = \frac{\sqrt{x^2-25}}{(4-x^2)}+\log_{10}(x^2+2x-15),

we need to consider the domain conditions for both the square root function and the logarithmic function.

The square root function x225\sqrt{x^2-25} requires that the argument of the square root be non-negative, so

x2250.x^2 - 25 \geq 0.

This inequality is satisfied when

x5orx5.x \leq -5 \quad \text{or} \quad x \geq 5.

The denominator of the rational part of f(x)f(x), (4x2)(4-x^2), cannot be zero, otherwise, the function will become undefined due to division by zero.

Thus, we must have

4x20.4 - x^2 \neq 0.

This inequality is violated when

x=±2.x = \pm2.

Combining these conditions gives us the domain for the rational part of the function:

x(,5](5,)andx2,2.x \in (-\infty, -5] \cup (5, \infty) \quad \text{and} \quad x \neq 2,-2.

Moving on to the logarithmic function, log10(x2+2x15)\log_{10}(x^2+2x-15), the argument must be positive:

x2+2x15>0.x^2 + 2x - 15 > 0.

This is a quadratic inequality, which we can factor to find the solution:

(x+5)(x3)>0.(x+5)(x-3) > 0.

From this, we see that the inequality is satisfied for

x<5orx>3.x < -5 \quad \text{or} \quad x > 3.

The overall domain of f(x)f(x) is the intersection of the domains for each piece.

Taking the intersection of the two sets gives us:

x(,5)(5,),x \in (-\infty, -5) \cup (5, \infty),

Since the question states that the domain is of the form (,α)[β,)(-\infty, \alpha) \cup [\beta, \infty), we can infer that

α=5andβ=5.\alpha = -5 \quad \text{and} \quad \beta = 5.

We calculate α2+β3\alpha^2 + \beta^3 as follows:

α2+β3=(5)2+53=25+125=150.\alpha^2 + \beta^3 = (-5)^2 + 5^3 = 25 + 125 = 150.

So the correct answer, representing the sum of α2\alpha^2 and β3\beta^3, is: Option D 150150.

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