- #1
JohnSimpson
- 92
- 0
[tex]\sum{a^ncos(nx)}[/tex]
from zero to infinity
a is a real number -1 < a < 1
I rewrote this as a geometric series involving a complex exponential
Real part of
[tex]\sum{(ae^{ix})^n}[/tex]
Which is a geometric series with common ratio r < 1, so it converges to the sum
(first term)/(1-r)
which seems to be
[tex]\frac{1}{1-ae^{ix}}[/tex]
taking the real part and multiplying top and bottom by (1-acosx), I get[tex]\frac{1-acos(x)}{1-2acos(x) + a^2cos^2(x))}[/tex]
which is different from the desired result of
[tex]\frac{1-acos(x)}{1-2acos(x) + a^2)}[/tex]
Any help would be appreciated
from zero to infinity
a is a real number -1 < a < 1
I rewrote this as a geometric series involving a complex exponential
Real part of
[tex]\sum{(ae^{ix})^n}[/tex]
Which is a geometric series with common ratio r < 1, so it converges to the sum
(first term)/(1-r)
which seems to be
[tex]\frac{1}{1-ae^{ix}}[/tex]
taking the real part and multiplying top and bottom by (1-acosx), I get[tex]\frac{1-acos(x)}{1-2acos(x) + a^2cos^2(x))}[/tex]
which is different from the desired result of
[tex]\frac{1-acos(x)}{1-2acos(x) + a^2)}[/tex]
Any help would be appreciated