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Date May 2021 Marks available 2 Reference code 21M.2.AHL.TZ2.7
Level Additional Higher Level Paper Paper 2 Time zone Time zone 2
Command term Determine Question number 7 Adapted from N/A

Question

Consider the following system of coupled differential equations.

dxdt=-4x

dydt=3x-2y

Find the value of dydx

Find the eigenvalues and corresponding eigenvectors of the matrix -403-2.

[6]
a.

Hence, write down the general solution of the system.

[2]
b.

Determine, with justification, whether the equilibrium point (0, 0) is stable or unstable.

[2]
c.

(i)   at (4, 0).

(ii)  at (-4, 0).

[3]
d.

Sketch a phase portrait for the general solution to the system of coupled differential equations for 6x6, 6y6.

[4]
e.

Markscheme

-4-λ03-2-λ=0            (M1)

-4-λ-2-λ=0            (A1)

λ=-4  OR  λ=-2            A1

λ=-4

-403-2xy=-4x-4y            (M1)


Note: This M1 can be awarded for attempting to find either eigenvector.


3x-2y=-4y

3x=-2y

possible eigenvector is -23 (or any real multiple)            A1

λ=-2

-403-2xy=-2x-2y

x=0, y=1

possible eigenvector is 01 (or any real multiple)            A1


[6 marks]

a.

xy=Ae-4t-23+Be-2t01            (M1)A1


Note:
Award M1A1 for x=-2Ae-4t, y=3Ae-4t+Be-2t, M1A0 if LHS is missing or incorrect.

 

[2 marks]

b.

two (distinct) real negative eigenvalues                    R1

(or equivalent (eg both e-4t0, e-2t0 as t))

⇒ stable equilibrium point                         A1


Note:
 Do not award R0A1.

 

[2 marks]

c.

dydx=3x-2y-4x                        (M1)

(i)    (4, 0)dydx=-34                        A1

(ii)   (-4, 0)dydx=-34                        A1

 

[3 marks]

d.

        A1A1A1A1

 

Note: Award A1 for a phase plane, with correct axes (condone omission of labels) and at least three non-overlapping trajectories. Award A1 for all trajectories leading to a stable node at (0, 0). Award A1 for showing gradient is negative at x=4 and -4. Award A1 for both eigenvectors on diagram.

 

[4 marks]

e.

Examiners report

[N/A]
a.
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b.
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c.
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d.
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e.

Syllabus sections

Topic 1—Number and algebra » AHL 1.15—Eigenvalues and eigenvectors
Show 60 related questions
Topic 5—Calculus » AHL 5.17—Phase portrait
Topic 1—Number and algebra
Topic 5—Calculus

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