Let ω be a nonreal root of z3=1. Let a1,a2,…,an be real numbers such that 1a1+ω+1a2+ω+⋯+1an+ω=2+5i.Compute 2a1−1a21−a1+1+2a2−1a22−a2+1+⋯+2an−1a2n−an+1.
ω=−12±i√32.
Consider a general term on the lhs,
1ak+ω=1(ak−1/2)±i√3/2=1(ak−1/2)±i√3/2.(ak−1/2)∓i√3/2(ak−1/2)±i√3/2
=(ak−1/2)∓i√3/2(ak−1/2)2+3/4=(ak−1/2)∓i√3/2a2k−ak+1.
Substitute and equate reals,
a1−1/2a21−a1+1+a2−1/2a22−a2+1+⋯+an−1/2a2n−an+1=2,
and multiplying that by 2,
2a1−1a21−a1+1+2a2−1a22−a2+1+⋯+2an−1a2n−an+1=4.
maybe this helps:
Letωbeanonrealrootofz3=1.Leta1,a2,…,anberealnumberssuchthat\[1a1+ω+1a2+ω+⋯+1an+ω=2+5i.\]Compute\[2a1−1a21−a1+1+2a2−1a22−a2+1+⋯+2an−1a2n−an+1.\]
ω=−12±i√32.
Consider a general term on the lhs,
1ak+ω=1(ak−1/2)±i√3/2=1(ak−1/2)±i√3/2.(ak−1/2)∓i√3/2(ak−1/2)±i√3/2
=(ak−1/2)∓i√3/2(ak−1/2)2+3/4=(ak−1/2)∓i√3/2a2k−ak+1.
Substitute and equate reals,
a1−1/2a21−a1+1+a2−1/2a22−a2+1+⋯+an−1/2a2n−an+1=2,
and multiplying that by 2,
2a1−1a21−a1+1+2a2−1a22−a2+1+⋯+2an−1a2n−an+1=4.