` A = x^2 -10x + 3 = x^2 - 2*5*x + 25 - 22 = (x-5)^2 -22 \ge -22`
`=> A_(min) = -22` khi ` x -5 = 0 => x =5`
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` B = 5 - 2x -3` ?
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` C =-3x^2+5x +3 = - 3(x^2 -5/3 x + 25/36) + 3 + 3*25/36`
` = -3(x-5/6)^2 + 61/12 \le 61/12`
` => C_(max) = 61/12` khi ` x -5/6 = 0 => x= 5/6`