Ta có: \(\frac{a}{b}=\frac{c}{d}\Rightarrow\frac{a}{c}=\frac{b}{d}=\frac{2015a}{2015c}=\frac{2016b}{2016d}\)

                                       \(=\frac{2015a-2016b}{2015c-2016d}=\frac{2015a+2016b}{2015c+2016d}\)

\(\Rightarrow\frac{2015a-2016b}{2015a+2016b}=\frac{2015c-2016d}{2015c+2016d}\)(đpcm)

Từ \(\frac{a}{b}=\frac{c}{d}\)ta suy ra:

\(\frac{a}{b}=\frac{c}{d}=\frac{a}{c}=\frac{b}{d}=\frac{a+b}{c+d}=\frac{a-b}{c-d}\Rightarrow\frac{a+b}{a-b}=\frac{c+d}{c-d}=\frac{a-b}{a+b}=\frac{c-d}{c+d}\Rightarrow\frac{2015a-2016b}{2015a+2016b}\)\(=\frac{2015c-2016d}{2015c+2016d}\)(Áp dụng tính chất dãy tỉ số bằng nhau)

Đặt a/b=c/d=k

=>a=bk; c=dk

\(\dfrac{2015a-2016b}{2016c+2017d}=\dfrac{2015bk-2016b}{2016dk+2017d}=\dfrac{2015k-2016}{2016k+2017}\)

\(\dfrac{2015c-2016d}{2016a+2017b}=\dfrac{2015dk-2016d}{2016bk+2017b}=\dfrac{2015k-2016}{2016k+2017}\)

Do đó: \(\dfrac{2015a-2016b}{2016c+2017d}=\dfrac{2015c-2016d}{2016a+2017b}\)

Ta có \(a^{14}+b^{14}=a^{15}+b^{15}\Leftrightarrow a^{15}-a^{14}=b^{14}-b^{15}\Leftrightarrow a^{14}\left(a-1\right)=b^{14}\left(1-b\right)\Leftrightarrow\dfrac{a-1}{1-b}=\dfrac{b^{14}}{a^{14}}\left(1\right)\)

ta lại có \(a^{15}+b^{15}=a^{16}+b^{16}\Leftrightarrow a^{16}-a^{15}=b^{15}-b^{16}\Leftrightarrow a^{15}\left(a-1\right)=b^{15}\left(1-b\right)\Leftrightarrow\dfrac{a-1}{b-1}=\dfrac{b^{15}}{a^{15}}\left(2\right)\)

Từ (1),(2)\(\Rightarrow\dfrac{b^{14}}{a^{14}}=\dfrac{b^{15}}{a^{15}}\Leftrightarrow\dfrac{b^{15}}{a^{15}}-\dfrac{b^{14}}{a^{14}}=0\Leftrightarrow\dfrac{b^{14}}{a^{14}}\left(\dfrac{a}{b}-1\right)=0\Leftrightarrow\dfrac{a}{b}-1=0\)(vì \(\dfrac{a^{14}}{b^{14}}\) là số dương)\(\Leftrightarrow\dfrac{a}{b}=1\Leftrightarrow a=b\)

Vậy thay vào P=2015a-2016b=2015a-2016a=-a=-b

Vậy P=-a=-b