The curve C has equation
y = 3x^3 - 8x^2 - 3
(a) (i) Find
dy/dx
(ii) d^2y/dx^2
(b) Verify that C has a stationary point when x = 2
(c) Determine the nature of this stationary point, giving a reason for your answer. - Edexcel - A-Level Maths Pure - Question 2 - 2017 - Paper 1
Question 2
The curve C has equation
y = 3x^3 - 8x^2 - 3
(a) (i) Find
dy/dx
(ii) d^2y/dx^2
(b) Verify that C has a stationary point when x = 2
(c) Determine the nature of ... show full transcript
Worked Solution & Example Answer:The curve C has equation
y = 3x^3 - 8x^2 - 3
(a) (i) Find
dy/dx
(ii) d^2y/dx^2
(b) Verify that C has a stationary point when x = 2
(c) Determine the nature of this stationary point, giving a reason for your answer. - Edexcel - A-Level Maths Pure - Question 2 - 2017 - Paper 1
Step 1
Find (i) dy/dx
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Answer
To find the first derivative, we differentiate the function:
dy/dx=dxd(3x3−8x2−3)=9x2−16x
Step 2
Find (ii) d^2y/dx^2
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Answer
Next, we find the second derivative by differentiating the first derivative:
d2y/dx2=dxd(9x2−16x)=18x−16
Step 3
Verify that C has a stationary point when x = 2
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Answer
We substitute x = 2 into the first derivative:
dy/dx=9(2)2−16(2)=36−32=4
Since dy/dx=0 is not satisfied, we need to check if it comes from a change of sign in the first derivative near this point for verification.
Step 4
Determine the nature of this stationary point, giving a reason for your answer
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Answer
Now, substituting x = 2 into the second derivative:
d2y/dx2=18(2)−16=36−16=20
Since d2y/dx2>0, it indicates that the stationary point at x = 2 is a minimum.