Q23 / 54 · JEE Advanced 2019 (Paper 2) · Mathematics

Let fx=sin⁡πxx2, x>0.
Let x1<x2<x3<…<xn<… be all the points of local maximum of f and y1<y2<y3<…<yn<… be all the points of local minimum of f.
Then which of the following options is/are correct?

  • Correct answer
    A
    xn-yn>1 for every n
  • B
    x1<y1
  • Correct answer
    C
    xn∈2n, 2n+12 for every n
  • Correct answer
    D
    xn+1-xn>2 for every n

Solution

f'x=2xcos⁡πxπx2-tan⁡πxx4 …(i)
⇒ for maxima/minima f'x=0
⇒cos⁡πx=0orπx2=tan⁡πx
∵cos⁡πx≠0∵tan⁡πx will not be defined
∴  maxima/minima will occur
Where tan⁡πx=πx2
Figure
Case I:
i For example
In x∈2n, 2n+12 at point P2, x∈2,52
cos⁡πx>0
and at P2+, tan⁡πx>πx2
at P2-, tan⁡πx<πx2
hence from equation (i)
in x∈2n, 2n+12, f'x goes from positive to negative
hence P2 is maxima
Similarly P2, P4, P6 … are point of maxima and all lies in x∈2n, 2n+12
Case II
In x∈2n+1, 2n+32
ii for example at P1, x∈1,32
cos⁡πx<0
at P1+, tan⁡πx>πx2
and at P1-, tan⁡πx<πx2
hence from equation (i)
in x∈2n+1, 2n+32, f'x goes from negative to positive
⇒ so, for the minima y1:1<y1<32
y2:3<y2<72
y3:5<y3<112
and for the maxima x1:2<x1<52
x2:4<x2<92
x3:6<x3<132
Hence xn>yn … a
⇒  now,
x1>y1
tan⁡πx1>tan⁡πy1
tan⁡πx1>tan⁡π+πy1
tan⁡πx1>tan⁡π1+y1
πx1>π1+y1
x1>1+y1
Similarly, xn>1+yn
xn-yn>1 …(b)
xn-yn>1
⇒y2>x1
tan⁡πy2>tan⁡πx1
tan⁡πy2>tan⁡π+πx1⇒y2>x1+1
⇒yn+1>xn+1
Therefore xn<yn+1<xn+1⇒yn+1-xn>1 …(c)
Now consider xn+1-xn=xn+1-yn+1+yn+1-xn
From (b) and (c)
xn+1-xn>2

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

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