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Date May Example question Marks available 3 Reference code EXM.2.AHL.TZ0.22
Level Additional Higher Level Paper Paper 2 Time zone Time zone 0
Command term Show that Question number 22 Adapted from N/A

Question

Let M2 = M where M ( a b c d ) , b c 0

Show that  a + d = 1 .

[3]
a.i.

Find an expression for b c in terms of a .

[2]
a.ii.

Hence show that M is a singular matrix.

[3]
b.

If all of the elements of M are positive, find the range of possible values for a .

[3]
c.

Show that (IM)2 = IM where I is the identity matrix.

[3]
d.

Markscheme

Attempting to find M2           M1

M2 =  ( a 2 + b c a b + b d a c + c d b c + d 2 )            A1

b ( a + d ) = b or  c ( a + d ) = c            A1

Hence  a + d = 1    (as  b 0 or c 0 )      AG  N0

[3 marks]

a.i.

a 2 + b c = a         M1

b c = a a 2          A1  N1

[2 marks]

a.ii.

METHOD 1

Using det M = a d b c         M1

det M =  a d a ( 1 a ) or det M =  a ( 1 a ) a ( 1 a )

(or equivalent)         A1

       = 0 using  a + d = 1 or  d = 1 a  to simplify their expression         R1

Hence M is a singular matrix         AG  N0

 

METHOD 2

Using  b c = a ( 1 a ) and  a + d = 1 to obtain  b c = a d         M1A1

det M =  a d b c and  a d b c = 0 as  b c = a d          R1

Hence M is a singular matrix         AG  N0

 

[3 marks]

b.

a ( 1 a ) > 0        (M1)

0 <  a < 1        A1A1    N3

Note: Award A1 for correct endpoints and A1 for correct inequality signs.

[3 marks]

c.

METHOD 1

Attempting to expand (I − M)2      M1

(I − M)2 = I − 2M + M2      A1    

              = I − 2M + M          A1    

              = I − M         AG   N0

 

METHOD 2

Attempting to expand (I − M)2 =  ( 1 a b c 1 d ) 2   (or equivalent)      M1

(I − M)2 =  ( ( 1 a ) 2 + b c b ( 1 a ) b ( 1 d ) c ( 1 a ) c ( 1 d ) b c + ( 1 d ) 2 )

(or equivalent)          A1  

Use of a + d = 1 and b c = a a 2  to show desired result.      M1

Hence (I − M)2 =  ( 1 a b c 1 d )       AG   N0

 

[3 marks]

d.

Examiners report

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

Syllabus sections

Topic 1—Number and algebra » AHL 1.14—Introduction to matrices
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Topic 1—Number and algebra

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