Maths-
General
Easy

Question

A function y equals f left parenthesis x right parenthesis has a second‐order derivatives f to the power of ´ ´ end exponent left parenthesis x right parenthesis equals 6 left parenthesis x minus 1) If its graph passes through the point left parenthesis 2 comma blank 1 right parenthesis and at that point the tangent to the grraph i s y equals 3 x minus 5, then the function is

  1. left parenthesis x plus 1 right parenthesis to the power of 3 end exponent    
  2. left parenthesis x minus right square bracket right parenthesis to the power of 3 end exponent    
  3. left parenthesis x plus 1 right parenthesis to the power of 2 end exponent    
  4. left parenthesis x minus right square bracket right parenthesis to the power of 2 end exponent    

hintHint:

We are given a second derivative of a function. We are given the points through which the function is passing. And, we are given the equation of tangent at that point. We have to find the function. We will use integration to solve the question.

The correct answer is: left parenthesis x minus right square bracket right parenthesis to the power of 3 end exponent


    The given second order derivative of function is f"(x) = 6(x - 1). To find the first derivative, we will integrate the second derivative.
    integral f " left parenthesis x right parenthesis space equals space f apostrophe left parenthesis x right parenthesis space plus space C apostrophe
    We will integrate the second derivative.
    integral f " left parenthesis x right parenthesis space d x space equals space integral 6 left parenthesis x space minus space 1 right parenthesis space d x
space space space space space space space space space space space space space space space space space space space equals integral left parenthesis 6 x space minus space 6 right parenthesis space d x
space space space space space space space space space space space space space space space space space space space equals fraction numerator 6 x squared over denominator 2 end fraction minus 6 x space plus space C space space space space space... left parenthesis space C space i s space a space c o n s tan t right parenthesis

A s space i n t e g r a t i o n space o f space s e c o n d space o r d e r space d e r i v a t i v e space i s space f i r s t space
o r d e r space d e r i v a t i v e space w e space c a n space w r i t e comma
space f apostrophe left parenthesis x right parenthesis space equals space fraction numerator 6 x squared over denominator 2 end fraction space minus space 6 x space plus space C
    We know that the first derivative of a function at certain point gives us tangent at that point.
    The equation of tangent at the point of derivative is (y - y1) = f'(x)(x - x1)
    We will find the value of derivative at the point (2,1). We will substitute x = 2 in the first order derivative.
    f apostrophe left parenthesis x right parenthesis space equals space fraction numerator 6 x squared over denominator 2 end fraction space minus space 6 x space plus space C
f apostrophe left parenthesis 2 right parenthesis space equals space fraction numerator 6 left parenthesis 2 right parenthesis squared over denominator 2 end fraction space minus space 6 left parenthesis 2 right parenthesis space plus space C
space space space space space space space space space space space equals space 12 space minus space 12 space plus space C
space space space space space space space space space space space equals space C
f apostrophe left parenthesis 2 right parenthesis space equals space C
    We will substitute the values of f'(x) and the point in the equation of the tangent.
    (y - 1) = C(x - 2)
    y - 1 = Cx - 2C
    y = Cx - 2C + 1
    y = Cx - (2C - 1)
    Comparing with the given equation of tangent of the curve
    y = 3x - 5
    Cx = 3x
    So, C = 3
    We will substitute the value of C in the equation of first order derivative.
    f apostrophe left parenthesis x right parenthesis space equals fraction numerator 6 x squared over denominator 2 end fraction space minus space 6 x space plus space C
f apostrophe left parenthesis x right parenthesis space equals space fraction numerator 6 x squared over denominator 2 end fraction space minus space 6 x space plus space 3
space
    Now, we will integrate the first order derivative to get the value of function.
    y space equals space f left parenthesis x right parenthesis
space space space space equals integral f apostrophe left parenthesis x right parenthesis space d x
space space space equals integral left parenthesis fraction numerator 6 x squared over denominator 2 end fraction minus 6 x space plus space 3 right parenthesis space d x
space y space equals space fraction numerator 6 x cubed over denominator 6 end fraction minus space fraction numerator 6 x squared over denominator 2 end fraction plus 3 x space plus space H space space space space space space space space... left parenthesis space H space i s space a space c o n s tan t right parenthesis
    To find the value of the constant, we will substitute the value of the point (2,1) in the function.
    1 space equals space left parenthesis 2 right parenthesis cubed space minus space 3 left parenthesis 2 right parenthesis squared space plus 3 left parenthesis 2 right parenthesis space plus space H
1 space equals space 8 space minus space 12 space plus space 6 space plus space H
1 space equals space 2 space plus space H
R e a r r a g i n g space a n d space s u b t r a c t i n g space 2 space f r o m space b o t h space t h e space s i d e s
H space equals space 1 space minus space 2
H space equals space minus 1
    y = x3 - 3x2 + 3x - 1
    The given function is the expansion of formula (x - 1)3
    y = (x - 1)3
    This is the required function.

    For such questions, the important part is integration. We should know the methods to integrate the derivate. We should know the properties of a tangent.

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