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F.5 MATHS (不等式)......

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1.A gardener is going to put up 84m of fence around a rectangular plot of land. If the area to be enclosed is at least 425m2, find the maximum length of the plot. 沒有ANS 大家教教我點做

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1. A gardener is going to put up 84 m of fence around a rectangular plot of land. If the area to be enclosed is at least 425 m^2, find the maximum length of the plot. Let the length of the plot be x cm. The width of the plot is : (84-2x)/2 m = (42-x) m The area to be enclosed is : x(42-x) m^2 x(42-x)>425 x^2-42x+425<0 (x-17)(x-25)<0 case (i) : (x-17)<0 and (x-25)>0 x<17 and x>25 No solution case (ii) : (x-17)>0 and (x-25)<0 x>17 and x<25 17
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