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標題:

maths quadratic equations 請幫幫忙

發問:

1.Joan had bought a number of pens from a shop for $300 last week. The shop reduced the price of each pen by $0.3 this week. If Joan bought the pens this week, she would buy 20 more pens by paying extra $8. How much was a pen after the price reduction? 緊急!緊急!

最佳解答:

Let x be the price and q be the quantity purched in 300 X ． Q = 300 --------- (1) (X - 0.3) (Q + 20) = 308----------(2) (2) ====> XQ - 0.3Q + 20X - 6 = 308 --------- (3) Put (1) into (3) ====> 300 - 0.3Q + 20 X-6 = 308 Q = (20X - 14) / 0.3-------- (4) Put (4) into (1) (X) (20X - 14) / 0.3 = 300 20X^2 -14X = 90 20X^2 - 14 X -90=0 10X^2 - 7X - 45 = 0 (2x – 5)(5x + 18) = 0 X = 2.5 or – 3.6 (rejected) the reduced price is x – 0.3 = 2.2 2006-10-28 11:38:33 補充： the last steps should be 10X^2 - 7X - 45 = 0 (2x – 5)(5x + 9) = 0 X = 2.5 or – 1.8 (rejected) the reduced price is x – 0.3 = 2.2

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let x be the no. of pens she bought last week. let y be the price of each pen after price reduction. x(y+0.3)=300 (x+20)y-300=8 by solving the two equations,you can find the answer.7638E7481407D16B