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∈ℂ.
Two of the roots of this equation are a+bi and b+ai, where a, b∈ℤ.
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+b−ai=−4
2a+2b=−4(⇒a+b=−2) A1
(a+bi)(a−bi) (b+ai)(b−ai)=400
(a2+b2)2=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+bi))(z-(a-bi))(z-(b+ai))(z-(b-ai))(=0) A1
((z-a)2+b2)((z-b)2+a2)(=0)
(z2-2az+a2+b2)(z2-2bz+b2+a2)(=0) A1
Attempt to equate coefficient of z3 and constant with the given quartic equation M1
-2a-2b=4 and (a2+b2)2=400 A1
attempt to solve simultaneous equations (M1)
a=2 or a=-4 A1A1
[8 marks]