Question

Cho đa thức P(x) = \(\frac{2^{2x+1}}{2^{2x}-2}\) với x \(\ne\) \(\frac{1}{2}\)

a, CMR: P(k) + P(1 - k) = 2 với mọi k\(\ne\)1

b, Tính giá trị của biểu thức \(A=2009+P\left(\frac{1}{2009}\right)+P\left(\frac{2}{2009}\right)+...+P\left(\frac{2008}{2009}\right)\)

Answer 1

\(P\left(k\right)+P\left(1-k\right)=\frac{2^{2k+1}}{2^{2k}-2}+\frac{2^{2\left(1-k\right)+1}}{2^{2\left(1-k\right)}-2}=\frac{2^{2k+1}}{2^{2k}-2}+\frac{2^{3-2k}}{2^{2-2k}-2}\)

\(=\frac{2^{2k+1}}{2^{2k}-2}+\frac{2^2}{2-2^{2k}}=\frac{2^{2k+1}}{2^{2k}-2}-\frac{4}{2^{2k}-2}=\frac{2\left(2^{2k}-2\right)}{2^{2k}-2}=2\) (đpcm)

Áp dụng cho câu b:

\(A=2009+P\left(\frac{1}{2009}\right)+P\left(\frac{2008}{2009}\right)+P\left(\frac{2}{2009}\right)+P\left(\frac{2007}{2009}\right)+...+P\left(\frac{1004}{2009}\right)+P\left(\frac{1005}{2009}\right)\)

\(=2009+P\left(\frac{1}{2009}\right)+P\left(1-\frac{1}{2009}\right)+...+P\left(\frac{1004}{2009}\right)+P\left(1-\frac{1004}{2009}\right)\)

\(=2009+2+2+...+2\) (có 1004 số 2)

\(=2009+2.1004=4017\)