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Date November 2019 Marks available 6 Reference code 19N.2.AHL.TZ0.H_6
Level Additional Higher Level Paper Paper 2 Time zone Time zone 0
Command term Find Question number H_6 Adapted from N/A

Question

Let P ( z ) = a z 3 37 z 2 + 66 z 10 , where  z C and  a Z .

One of the roots of  P ( z ) = 0 is  3 + i . Find the value of a .

Markscheme

METHOD 1

one other root is  3 i          A1

let third root be  α        (M1)

considering sum or product of roots       (M1)

sum of roots  = 6 + α = 37 a          A1

product of roots  = 10 α = 10 a          A1

hence  a = 6          A1

 

METHOD 2

one other root is  3 i          A1

quadratic factor will be  z 2 6 z + 10        (M1)A1

P ( z ) = a z 3 37 z 2 + 66 z 10 = ( z 2 6 z + 10 ) ( a z 1 )        M1

comparing coefficients       (M1)

hence  a = 6          A1

 

METHOD 3

substitute  3 + i into  P ( z )        (M1)

a ( 18 + 26 i ) 37 ( 8 + 6 i ) + 66 ( 3 + i ) 10 = 0        (M1)A1

equating real or imaginary parts or dividing       M1

18 a 296 + 198 10 = 0   or   26 a 222 + 66 = 0   or   10 66 ( 3 + i ) + 37 ( 8 + 6 i ) 18 + 26 i          A1

hence  a = 6          A1

 

[6 marks]

Examiners report

[N/A]

Syllabus sections

Topic 1—Number and algebra » AHL 1.14—Complex roots of polynomials, conjugate roots, De Moivre’s, powers & roots of complex numbers
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