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  1. Let V be a linear space with norm ∥·∥, W a subspace of V, and f ∈ V. Prove that the set of best approximations to f by elements in W is convex.
  2. Find the best uniform approximation to f(x) = sin2x on [0,2π] by polynomials of degree at most 2.
  3. Let f ∈ C[a, b]. Find the best uniform approximation to f by a constant.
  4. Let V = R3 with ∥·∥∞ and W = span{(0,1,0),(0,0,1)}. Let f = (3,6,4). Prove that
    a best approximation to f is not unique.
  5. Prove that every p ∈ Pn has a unique representation of the form
    p(x)=a0 +a1T1(x)+…+anTn(x), (1) where Tj , for j = 1, . . . , n are the Chebyshev polynomials of degree j.
  6. Plot T1, T2, T3 and T6.