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Question

Number of points where f(x)=[x]sin2(πx) is not differentiable if x(7,10) is (where [.] denotes the greatest integer function)

A
15
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B
0
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C
6
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D
17
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Solution

The correct option is B 0
f(x)=[x]sin2(πx)
f(x)=⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪7sin2(πx),x(7,6)0,x=66sin2(πx),x(6,5)0,x=50,x=99sin2(πx),x(9,10)
Clearly f(x) is continuous for x(7,10)

f(x)=⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪ ⎪7sin(2πx),x(7,6)0,x=66sin(2πx),x(6,5)0,x=50,x=99sin(2πx),x(9,10)
Clearly f(x) is differentiable for x(7,10)
Hence, there is no such point where f(x) is not differentiable for x(7,10)

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