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Date May 2019 Marks available 4 Reference code 19M.1.AHL.TZ0.F_3
Level Additional Higher Level Paper Paper 1 Time zone Time zone 0
Command term Show that Question number F_3 Adapted from N/A

Question

The matrix A is given by  = [ a b c d ] .

The matrix B is given by B  = [ 3 2 2 3 ] .

Show that the eigenvalues of A are real if  ( a d ) 2 + 4 b c 0 .

[4]
a.i.

Deduce that the eigenvalues are real if A is symmetric.

[2]
a.ii.

Determine the eigenvalues of B.

[2]
b.i.

Determine the corresponding eigenvectors.

[4]
b.ii.

Markscheme

the eigenvalues satisfy

| a λ b c d λ | = 0      M1

( a λ ) ( d λ ) b c = 0       A1

λ 2 ( a + d ) λ + a d b c = 0       A1

the condition for real roots is 

( a + d ) 2 4 ( a d b c ) 0       M1

( a d ) 2 + 4 b c 0       AG

[4 marks]

a.i.

if the matrix is symmetric, b = c. In this case,       M1

( a d ) 2 + 4 b c = ( a d ) 2 + 4 b 2 0

because each square term is non-negative      R1AG

[2 marks]

a.ii.

the characteristic equation is

λ 2 6 λ + 5 = 0      M1

λ = 1 , 5       A1

[2 marks]

b.i.

taking  λ = 1

[ 2 2 2 2 ] [ x y ] = [ 0 0 ]      M1

giving eigenvector  = [ 1 1 ]        A1

 

taking  λ = 5

[ 2 2 2 2 ] [ x y ] = [ 0 0 ]      M1

giving eigenvector  = [ 1 1 ]        A1

[4 marks]

b.ii.

Examiners report

[N/A]
a.i.
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a.ii.
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b.i.
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b.ii.

Syllabus sections

Topic 1—Number and algebra » AHL 1.15—Eigenvalues and eigenvectors
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