Q25 / 54 · JEE Advanced 2019 (Paper 1) · Mathematics

Let f:R→R be given by f x =x5+5x4+10x3+10x2+3x+1,x<0;x2-x+1,0≤x<1;23x3-4x2+7x-83,1≤x<3;x-2logex-2-x+103,x≥3
Then which of the following options is/are correct?

  • Correct answer
    A
    f′ is NOT differentiable at x=1
  • B
    f is increasing on -∞, 0
  • Correct answer
    C
    f is onto
  • Correct answer
    D
    f′ has a local maximum at x=1

Solution

As given f:R→R
f x =x5+5x4+10x3+10x2+3x+1x<0x2-x+10≤x<123x3-4x2+7x-831≤x<3(x-2)ln(x-2)-x+103x≥3
 f(0)=f(0+)=f(0)=1
 f1=f1+=f1=1
 f3=f3+=f3=13
Hence fx is continuous everywhere.
Now, limx→∞fx=-∞ and limx→∞fx=+∞
Hence, range of f(x) is -∞,∞
Consider f'x =5x4+20x3+30x2+20x+3=5(x+1)4-2x<02x-10<x<12x2-8x+71<x<3ln(x-2)x>3
Option (B):
For x<0 f′x=5x+14 -2, which takes both positive and negative values.
Hence, f(x) is non-monotonic for x<0
Option (A):
f ″ x =20(x+1)3x<020<x<14x-31<x<31x-2x>3
Now, f”1′= 4<0 and f”1-=2>0
Given of f′(x) in x∈0,3
Figure
x=1 is maxima for f′x -----Option D
and f′x is not differential at x=1
Option (C):
limx→∞fx=limx→∞x-2ln⁡x-2-x+103
limx→∞fx=limx→∞x-2ln⁡x-2-1+43=∞
limx→-∞fx=-∞
Hence, range of f  ∈ -∞, ∞

Hence, the correct answer is options: 'A', 'C', 'D'.

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