let U_n = ∫_0 ^∞ ((cos(nx))/((x^2 +n^2 )^3 ))dx with n≥1
1) calculate U_n intrems of n
2) find lim_(n→+∞) n U_n
3) calculate lim_(n→+∞) n^2 U_n
4) study the convervence of U_n
A cube of unit edge length
is held before a plane. Prove that
the sum of the squares of the
projected lengths of edges of the cube
on the plane (irrespective of
the orientation of the cube) is 8.