` x^2 + 5x = x^2 + 2*5/2 *x + 25/4 - 25/4`
` = (x+5/2)^2 -25/4 \ge -25/4`
` => A_(min) = -25/4` khi ` x+5/2 = 0 => x= -5/2`
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` B = x^2 +4x + 3 = x^2 +4x +4 - 1 = (x+2)^2 -1 \ge -1`
` => B_(min) = -1`khi ` x +2 =0 => x =-2`
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` C = -x^2 -8x +5 = -(x^2 +8x-5) = -(x^2+ 8x +16) + 21 = -(x+4)^2 +16 \le 16`
` =>C_(max) = 16` khi ` x +4 = 0 => x= -4`
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`D = (x-1)(x+2)(x+3)(x+6) = (x^2 +5x -6)(x^2 +5x+6) = (x^2 +5x)^2 -36 \ge -36`
` => D_(min) = -36` khi ` x^2+ 5x = 0 => x \in {-5;0}`
