User interface language: English | Español

Date May 2021 Marks available 8 Reference code 21M.1.AHL.TZ1.7
Level Additional Higher Level Paper Paper 1 (without calculator) Time zone Time zone 1
Command term Find Question number 7 Adapted from N/A

Question

Consider the quartic equation z4+4z3+8z2+80z+400=0, z.

Two of the roots of this equation are a+bi and b+ai, where a, b.

Find the possible values of a.

Markscheme

METHOD 1

other two roots are a-bi and b-ai        A1

sum of roots =-4 and product of roots =400       A1

attempt to set sum of four roots equal to -4 or 4 OR
attempt to set product of four roots equal to 400        M1

a+bi+a-bi+b+ai+bai=4

2a+2b=4(a+b=2)         A1

(a+bi)(abi) (b+ai)(bai)=400

a2+b22=400         A1

a2+b2=20

attempt to solve simultaneous equations             (M1)

a=2 or a=-4           A1A1

 

METHOD 2

other two roots are a-bi and b-ai        A1

z-a+biz-a-biz-b+aiz-b-ai=0        A1

z-a2+b2z-b2+a2=0

z2-2az+a2+b2z2-2bz+b2+a2=0        A1

Attempt to equate coefficient of z3 and constant with the given quartic equation        M1

-2a-2b=4 and a2+b22=400        A1

attempt to solve simultaneous equations             (M1)

a=2 or a=-4           A1A1

 

[8 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
Show 55 related questions
Topic 1—Number and algebra

View options