f(x) = -2x² + 4x - 3 = -2(x - 1)² - 1
①x∈[a ,2]
a < 0 ：-2a² + 2a - 3 ≤ f(x) ≤ -1
0 ≤ a ≤1 ：-3 ≤ f(x) ≤ -1
a > 1 ：-3 ≤ f(x) ≤ -2a² + 2a - 3
②x∈[-2,-1]
-19 ≤ f(x) ≤ -9
③x∈[1/2 ,2]
-3 ≤ f(x) ≤ -1