设f(x)=ax
2+8x+3(a∈R).
(1)若g(x)=x•f(x),f(x)与g(x)在x同一个值时都取极值,求a;
(2)对于给定的负数a,当a≤-8时有一个最大的正数M(a),使得x∈[0,M(a)]时,恒有|f(x)|≤5.
(i)求M(a)的表达式;
(ii)求M(a)的最大值及相应的a的值.
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