Đặt \(f\left(x\right)=\left(x+1\right)P\left(x\right)-x\).
Khi đó \(f\left(k\right)=0\)với mọi \(k=0,1,2,...,2018\)mà \(P\left(x\right)\)có bậc \(2018\)nên \(f\left(x\right)\)có bậc \(2019\)
mà \(f\left(x\right)=0\)tại \(2019\)giá trị nên \(f\left(x\right)=ax\left(x-1\right)\left(x-2\right)...\left(x-2018\right)\).
Với \(x=-1\): \(a.\left(-1\right)\left(-2\right)...\left(-2019\right)=\left(-1+1\right)P\left(-1\right)-\left(-1\right)\)
\(\Leftrightarrow a=-\frac{1}{2019!}\).
\(P\left(2019\right)=\frac{f\left(2019\right)+2019}{2020}=\frac{-1+2019}{2020}=\frac{1009}{1010}\)
