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

Application of Derivatives question

2010 · Shift 0 · Q30
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. /Application of Derivatives
  5. /2010 · Shift 0 · Q30

Application of Derivatives question

2010 · Shift 0 · Q30

JEE MainMathematicsApplication of DerivativesMCQ+4 / −1
Let f:R→Rf:R \to Rf:R→R be defined by f(x)={k−2x,  ifx≤−12x+3,  ifx>−1f\left( x \right) = \left\{ {\begin{matrix} {k - 2x,\,\,if} & {x \le - 1} \\ {2x + 3,\,\,if} & {x \gt - 1} \\ \end{matrix} } \right.f(x)={k−2x,if2x+3,if​x≤−1x>−1​ If fff has a local minimum at x=−1x=-1x=−1, then a possible value of kkk is
  1. A
    000
  2. B
    −12- {1 \over 2}−21​
  3. C
    −1-1−1
  4. D
    111
View written solutionFree

Correct answer: C

  1. Given function
f(x)={k−2x,x≤−12x+3,x>−1f(x)= \begin{cases} k-2x, & x\le -1 \\ 2x+3, & x>-1 \end{cases}f(x)={k−2x,2x+3,​x≤−1x>−1​

We want fff to have a local minimum at x=−1x=-1x=−1.


  1. Value of the function at x=−1x=-1x=−1

Since the first branch includes x=−1x=-1x=−1,

f(−1)=k−2(−1)=k+2.f(-1)=k-2(-1)=k+2.f(−1)=k−2(−1)=k+2.
  1. Behavior on the left and right of x=−1x=-1x=−1
  • For x≤−1x\le -1x≤−1, f(x)=k−2x,f(x)=k-2x,f(x)=k−2x, which has slope −2<0-2<0−2<0. So as xxx increases toward −1-1−1 from the left, f(x)f(x)f(x) decreases. Hence for x<−1x<-1x<−1 close to −1-1−1, f(x)>f(−1).f(x)>f(-1).f(x)>f(−1).

  • For x>−1x>-1x>−1, f(x)=2x+3,f(x)=2x+3,f(x)=2x+3, which has slope 2>02>02>0. So as xxx increases to the right of −1-1−1, f(x)f(x)f(x) increases. Therefore near −1-1−1 on the right, the smallest nearby value on that branch is the right-hand limit: lim⁡x→−1+f(x)=2(−1)+3=1.\lim_{x\to -1^+}f(x)=2(-1)+3=1.limx→−1+​f(x)=2(−1)+3=1.

For x=−1x=-1x=−1 to be a local minimum, we need nearby values on both sides to be at least f(−1)f(-1)f(−1). The left side already satisfies this automatically. So we only need

1≥f(−1)=k+2.1\ge f(-1)=k+2.1≥f(−1)=k+2.

Thus,

k+2≤1  ⟹  k≤−1.k+2\le 1 \implies k\le -1.k+2≤1⟹k≤−1.
  1. Check the options
  • A: k=0k=0k=0 k+2=2>1k+2=2>1k+2=2>1 Right-side nearby values are less than f(−1)f(-1)f(−1), so not a local minimum.

  • B: k=−12k=-\frac12k=−21​ k+2=32>1k+2=\frac32>1k+2=23​>1 Not a local minimum.

  • C: k=−1k=-1k=−1 k+2=1k+2=1k+2=1 Then f(−1)=1f(-1)=1f(−1)=1, and nearby values on both sides are ≥1\ge 1≥1. So this works.

  • D: k=1k=1k=1 k+2=3>1k+2=3>1k+2=3>1 Not a local minimum.


  1. Conclusion

A possible value of kkk is

−1\boxed{-1}−1​

So the correct option is C.

PreviousNext

More from Application of Derivatives

  • Let f:R→R be a continuous function defined by f(x)=ex+2e−x1​ Statement - 1 : f(c)=31​, for some c∈R. Statement - 2 : 0<f(x)≤22​1​,…2010 · MCQ
  • Given P(x)=x4+ax3+bx2+cx+d such that x=0 is the only real root of P′(x)=0. If P(−1)<P(1), then in the interval [−1,1]:2009 · MCQ
  • Suppose the cubic x3−px+q has three distinct real roots where p>0 and q>0. Then which one of the following holds?2008 · MCQ
  • How many real solutions does the equation x7+14x5+16x3+30x−560=0 have?2008 · MCQ
  • If p and q are positive real numbers such that p2+q2=1, then the maximum value of (p+q) is2007 · MCQ
  • The function f(x)=tan−1(sinx+cosx) is an incresing function in2007 · MCQ
  • The function f(x)=2x​+x2​ has a local minimum at2006 · MCQ
  • A triangular park is enclosed on two sides by a fence and on the third side by a straight river bank. The two sides having fence are of same length x. The maximum area enclosed by the park is2006 · MCQ