Sign in
12thPass logo
New chatPYQ LibraryDoubtsRank report
Sign in to see Recents

Your guest activity stays on this device

Sign in to save progress →
Sign in

Limits Continuity and Differentiability question

2020 · 2 Sep · Shift 1 · Q23
Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
  1. PYQ Library
  2. /JEE Main
  3. /Mathematics
  4. /Limits Continuity and Differentiability
  5. /2020 · 2 Sep · Shift 1 · Q23

Limits Continuity and Differentiability question

2020 · 2 Sep · Shift 1 · Q23

JEE MainMathematicsLimits Continuity and DifferentiabilityMCQ+4 / −1
If a function f(x) defined by f(x)={aex+be−x,−1≤x<1cx2,1≤x≤3ax2+2cx,3<x≤4f\left( x \right) = \left\{ {\begin{matrix} {a{e^x} + b{e^{ - x}},} & { - 1 \le x \lt 1} \\ {c{x^2},} & {1 \le x \le 3} \\ {a{x^2} + 2cx,} & {3 \lt x \le 4} \\ \end{matrix} } \right.f(x)=⎩⎨⎧​aex+be−x,cx2,ax2+2cx,​−1≤x<11≤x≤33<x≤4​ be continuous for some aaa, b, c ∈\in∈ R and f'(0) + f'(2) = e, then the value of of aaa is :
  1. A
    ee2−3e−13{e \over {{e^2} - 3e - 13}}e2−3e−13e​
  2. B
    1e2−3e+13{1 \over {{e^2} - 3e + 13}}e2−3e+131​
  3. C
    ee2−3e+13{e \over {{e^2} - 3e + 13}}e2−3e+13e​
  4. D
    ee2+3e+13{e \over {{e^2} + 3e + 13}}e2+3e+13e​
View written solutionFree

Correct answer: C

  1. Given piecewise function
f(x)={aex+be−x,−1≤x<1cx2,1≤x≤3ax2+2cx,3<x≤4f(x)= \begin{cases} ae^x+be^{-x}, & -1\le x<1\\[4pt] cx^2, & 1\le x\le 3\\[4pt] ax^2+2cx, & 3<x\le 4 \end{cases}f(x)=⎩⎨⎧​aex+be−x,cx2,ax2+2cx,​−1≤x<11≤x≤33<x≤4​

We are told that f(x)f(x)f(x) is continuous and

f′(0)+f′(2)=e.f'(0)+f'(2)=e.f′(0)+f′(2)=e.

We need to find aaa.


  1. Use continuity at x=1x=1x=1

For continuity at x=1x=1x=1,

lim⁡x→1−f(x)=f(1).\lim_{x\to 1^-}f(x)=f(1).limx→1−​f(x)=f(1).

From the first branch,

lim⁡x→1−f(x)=ae+be−1.\lim_{x\to 1^-}f(x)=ae+b e^{-1}.limx→1−​f(x)=ae+be−1.

From the second branch,

f(1)=c(1)2=c.f(1)=c(1)^2=c.f(1)=c(1)2=c.

So,

ae+be=c...(1)ae+\frac{b}{e}=c \qquad ...(1)ae+eb​=c...(1)


  1. Use continuity at x=3x=3x=3

For continuity at x=3x=3x=3,

f(3)=lim⁡x→3+f(x).f(3)=\lim_{x\to 3^+}f(x).f(3)=limx→3+​f(x).

From the second branch,

f(3)=c(3)2=9c.f(3)=c(3)^2=9c.f(3)=c(3)2=9c.

From the third branch,

lim⁡x→3+f(x)=a(3)2+2c(3)=9a+6c.\lim_{x\to 3^+}f(x)=a(3)^2+2c(3)=9a+6c.limx→3+​f(x)=a(3)2+2c(3)=9a+6c.

Hence,

9c=9a+6c9c=9a+6c9c=9a+6c

3c=9a3c=9a3c=9a

c=3a....(2)c=3a. \qquad ...(2)c=3a....(2)


  1. Find derivatives needed

At x=0x=0x=0

Since 0∈[−1,1)0\in[-1,1)0∈[−1,1), use the first branch:

f(x)=aex+be−xf(x)=ae^x+be^{-x}f(x)=aex+be−x

Differentiate:

f′(x)=aex−be−xf'(x)=ae^x-be^{-x}f′(x)=aex−be−x

Thus,

f′(0)=a−b....(3)f'(0)=a-b. \qquad ...(3)f′(0)=a−b....(3)

At x=2x=2x=2

Since 2∈[1,3]2\in[1,3]2∈[1,3], use the second branch:

f(x)=cx2f(x)=cx^2f(x)=cx2

Differentiate:

f′(x)=2cxf'(x)=2cxf′(x)=2cx

Thus,

f′(2)=4c....(4)f'(2)=4c. \qquad ...(4)f′(2)=4c....(4)

Given

f′(0)+f′(2)=e,f'(0)+f'(2)=e,f′(0)+f′(2)=e,

so using (3) and (4),

a−b+4c=e....(5)a-b+4c=e. \qquad ...(5)a−b+4c=e....(5)


  1. Substitute c=3ac=3ac=3a into continuity equation

From (1):

ae+be=c=3a.ae+\frac{b}{e}=c=3a.ae+eb​=c=3a.

Multiply by eee:

ae2+b=3aeae^2+b=3aeae2+b=3ae

b=3ae−ae2=a(3e−e2)....(6)b=3ae-ae^2=a(3e-e^2). \qquad ...(6)b=3ae−ae2=a(3e−e2)....(6)


  1. Use derivative condition

From (5):

a−b+4c=ea-b+4c=ea−b+4c=e

Using c=3ac=3ac=3a,

a−b+12a=ea-b+12a=ea−b+12a=e

13a−b=e....(7)13a-b=e. \qquad ...(7)13a−b=e....(7)

Now substitute b=a(3e−e2)b=a(3e-e^2)b=a(3e−e2) from (6):

13a−a(3e−e2)=e13a-a(3e-e^2)=e13a−a(3e−e2)=e

a(13−3e+e2)=ea\big(13-3e+e^2\big)=ea(13−3e+e2)=e

a(e2−3e+13)=ea(e^2-3e+13)=ea(e2−3e+13)=e

Therefore,

a=ee2−3e+13.a=\frac{e}{e^2-3e+13}.a=e2−3e+13e​.


  1. Match with options

a=ee2−3e+13a=\frac{e}{e^2-3e+13}a=e2−3e+13e​

This is Option C.


  1. Comparison with stored answer

Stored correct answer: C

Our derived answer: C

So they agree.

PreviousNext

More from Limits Continuity and Differentiability

  • If x→1lim​x−1x+x2+x3+...+xn−n​= 820, (n ∈ N) then the value of n is equal to ​.2020 · Numerical
  • x→0lim​(tan(4π​+x))x1​ is equal to :2020 · MCQ
  • Let [t] denote the greatest integer ≤ t. If for some λ∈ R - {1, 0}, x→0lim​​λ−x+[x]1−x+∣x∣​​ = L, then L is equal to :2020 · MCQ
  • If x→0lim​{x81​(1−cos2x2​−cos4x2​+cos2x2​cos4x2​)} = 2-k then the value of k is ​…2020 · Numerical
  • x→alim​(3a+x)31​−(4x)31​(a+2x)31​−(3x)31​​ (ae 0) is equal to :2020 · MCQ
  • Suppose a differentiable function f(x) satisfies the identity f(x+y) = f(x) + f(y) + xy2 + x2y, for all real x and y. x→0lim​xf(x)​=1, then f'(3) is equal to ​.2020 · Numerical
  • Let f:(0,∞)→(0,∞) be a differentiable function such that f(1) = e and t→xlim​t−xt2f2(x)−x2f2(t)​=0. If f(x) = 1, then x is equal…2020 · MCQ
  • The function f(x)={4π​+tan−1x,21​(∣x∣−1),​∣x∣≤1∣x∣>1​ is :2020 · MCQ