解由x^2+y^2≤1设x=ksina,y=kcosa故k^2sin^2a+k^2cos^2a≤1即k^2≤1即-1≤k≤1则z=xy=ksinakcosa=k^21/2×2sinacosa=1/2k^2sin2a由-1≤sin2a≤1即-1/2≤1/2sin2a≤1/2又由k^2≤1即-1/2k^2≤1/2k^2sin2a≤1/2k^2即-1/...