What is the value of:
n∑k=1((−1)n−k(n+1k+1)(√51+√5(3+√5)k−√51−√5(3−√5)k−52))?
I am stuck because of the binomial coefficient there, because without it the sum would just be a bunch of geometric series.
Answer
∑nk=1((−1)n−k(n+1k+1)(√51+√5(3+√5)k−√51−√5(3−√5)k−52))?
Here's a start.
I'm feeling too tired right now
to do more.
∑nk=1(n+1k+1)xk=∑n+1k=2(n+1k)xk−1=1x∑n+1k=2(n+1k)xk=1x(−1−(n+1)x+∑n+1k=0(n+1k)xk)=1x((1+x)n+1−1−(n+1)x)
Note that
(3−√5)(3+√5)=4.
If x=−(3+√5),
∑nk=1(n+1k+1)(−1)k(3+√5)k=1−(3+√5)((1−(3+√5))n+1−1+(n+1)(3+√5))=−(3−√5)4((−1)n+1(2+√5)n+1−1+(n+1)(3+√5))
That's all.
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