Homework Statement
show that (U\bot)\bot=U, if (ℝ,V,+,[,.,]) an Euclidian space en U is a linear subspace of V.
Homework Equations
The Attempt at a Solution
suppose \beta={u_1,...,u_k} is an orthonormal basis of U.
pick u in U. Then u=x_1u_1+...+x_ku_k for certain x_1,...,x_k...