$$x^u+1=(x+1)( x^{u-1}-x^{u-2}+x^{u-3}+ \cdots +x^2-x^1+x^0)$$
$$x^u+1= x \cdot ( x^{u-1}-x^{u-2}+x^{u-3}+ \cdots +x^2-x^1+x^0) + ( x^{u-1}-x^{u-2}+x^{u-3}+ \cdots +x^2-x^1+x^0) $$
$$x^u+1= ( x^{u}-x^{u-1}+x^{u-2}+ \cdots +x^3-x^2+x^1) + ( x^{u-1}-x^{u-2}+x^{u-3}+ \cdots +x^2-x^1+x^0) $$
$$x^u+1= x^{u}+(-x^{u-1}+x^{u-2}+ \cdots +x^3-x^2+x^1 + x^{u-1}-x^{u-2}+x^{u-3}+ \cdots +x^2-x^1)+x^0 $$
$$(-x^{u-1}+x^{u-2}+ \cdots +x^3-x^2+x^1 + x^{u-1}-x^{u-2}+x^{u-3}+ \cdots +x^2-x^1)=0$$
$$x^u+1= x^{u}+x^0 $$
q.e.d.