y=x power 2 + 2x when x changes 2 to 1.8
Like so:
\(\delta x=1.8-2=-0.2 \\\\ \delta y=x_1^2-x_2^2+2(x_1-x_2)=(x_1-x_2)(x_1+x_2)+2(x_1-x_2)\\=\delta x(x_1+x_2)+2\delta x=\delta x(x_1+x_2+2)=-0.2(1.8+2+2)=-1.16 \)
