Ta có: C=5x(x+1)-5(x+y)(x-y)-5y(y+1)
C=5(\(x^2+x-\left(x+y\right)\left(x-y\right)-y^2-y\))
C=5(\(x^2-y^2+x-y-\left(x^2-y^2\right)\))
C=5(x-y)=5(2019--1)=5.2020=10100
=>5x^2+5x-5(x^2-y^2)-5y^2-5y
=>5x^2+5x-5x^2+5y^2-5y^2-5y
=>(5x^2-5x^2)+(5y^2-5y^2)+(5x-5y)
=>5(x-y)
thay x=2019,y=-1 vào biểu thức
=>5(2019-(-1))=5(2019+1)=5.2020=10100