f(x) = -x^2+4x = 4-(x-2)^2
max f(x)= f(2) =4
f(5)=-25+20=-5
-5≤f(m)≤2
-5≤-m^2+4m≤2
-5≤-m^2+4m and -m^2+4m≤2
m^2-4m-5≤0 and m^2-4m-2≥0
(m-5)(m+1)≤0 and "m≤ 2-√6 or m≥2+√6 "
-1≤m≤5 and "m≤ 2-√6 or m≥2+√6 "
-1≤m≤2-√6 or 2+√6≤m≤ 5
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如图
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