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The curve with equation y = f(x) passes through the point (1, 6) - Edexcel - A-Level Maths Pure - Question 10 - 2006 - Paper 1

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The curve with equation y = f(x) passes through the point (1, 6). Given that f'(x) = 3 + rac{5x^2 + 2}{x^3}, \, x > 0, find f(x) and simplify your answer.

Worked Solution & Example Answer:The curve with equation y = f(x) passes through the point (1, 6) - Edexcel - A-Level Maths Pure - Question 10 - 2006 - Paper 1

Step 1

Find f(x)

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Answer

To find f(x), we start with the expression for f'(x):

f(x)=3+5x2+2x3f'(x) = 3 + \frac{5x^2 + 2}{x^3}

We can rewrite this by finding a common denominator:

f(x)=3+5x2+2x3=3+5x+2x3f'(x) = 3 + \frac{5x^2 + 2}{x^3} = 3 + \frac{5}{x} + \frac{2}{x^3}

Next, we integrate f'(x):

f(x)=(3+5x+2x3)dxf(x) = \int (3 + \frac{5}{x} + \frac{2}{x^3}) \, dx

Carrying out the integration term by term, we have:

  • The integral of 3 is 3x.
  • The integral of \frac{5}{x} is 5 \ln |x|.
  • The integral of \frac{2}{x^3} is -\frac{2}{2x^2} = -\frac{1}{x^2}.

Thus,

f(x)=3x+5lnx1x2+Cf(x) = 3x + 5 \ln |x| - \frac{1}{x^2} + C

where C is the constant of integration.

Step 2

Use f(1) = 6 to find C

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Answer

We know that the curve passes through the point (1, 6), meaning:

f(1)=6f(1) = 6

Substituting x = 1 in f(x), we get:

6=3(1)+5ln(1)1+C6 = 3(1) + 5 \ln(1) - 1 + C

Since \ln(1) = 0, this simplifies to:

6=31+C6 = 3 - 1 + C

Therefore:

\Rightarrow C = 4.$$

Step 3

Write the final expression for f(x)

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Answer

Now substituting C back into f(x):

f(x)=3x+5lnx1x2+4.f(x) = 3x + 5 \ln |x| - \frac{1}{x^2} + 4.

Thus, the final simplified version of f(x) is:

f(x)=3x+5lnx1x2+4.f(x) = 3x + 5 \ln x - \frac{1}{x^2} + 4.

This is the required function.

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