F4 Maths Graphical Method 問題

問題係咁︰
The owner of a certain bakery estimates that if the selling price of a new type of cookies is set at $x each,then (180-20x0 pieces of cookies will be sold in a day , where 0< x <(下面有橫線)9.
a)Express, in term of x , the estimated daily income in selling the new type of cookies.
  咁ANSWERS 係  $(180x - 20"x二次方")
b)the figure shows the graph of y = -20"x二次方"+180x+100 for 0<(下面有橫線) x <(下面有橫線) 9. Use the figure to jude whether it is possible to earn a daily income of $500 in selling the new type of cookies.

咁我就計到y=100 ，而對個GRAPH係X=9，咁即係要賣出九件，但點解答案話係impossible呢？

仲有…
If the graphs of each pair of equations given below are drawn in the same rectangular coordinate plane ,what equation can be solved from the point(s) of intersection of the graphs?
a)y=3x=1,y=2"x二次方" - x - 3
  我計到係2"x二次方" - 4x - 4 =0，但答案係"x二次方" - 2x - 2 =0，點解GE？
b) y = -"x三次方" + 2 "x二次方" +1,y = 3x+1/2
  我計到-"x三次方"+ 2 "x二次方" - 3x +1/2 ，但答案係全卻乘二，點解GE？

唔該各位~~

原帖由 tomtom2015 於 2006-8-19 14:03 發表
a)y=3x=1,y=2"x二次方" - x - 3
  我計到係2"x二次方" - 4x - 4 =0，但答案係"x二次方" - 2x - 2 =0，點解GE？
b) y = -"x三次方" + 2 "x二次方" +1,y = 3x+1/2
  我計到-"x三次方"+ 2 "x二次方" - 3x +1/2 ，但答案係全卻乘二，點解GE？

a)   抽左個2出黎，再兩邊同時除 2，約走

b)   因為你個答案有1/2，所以整式乘2

[ 本帖最後由 arthur 於 2006-8-19 19:03 編輯 ]