100分求一道数学题,

题目

100分求一道数学题,
When 30 orange trees are planted per acre each tree yields 150 oranges For each additional tree per acre,the yield decreases by 3 oranges per tree.Express the total yield of oranges per acre,Y ,as a function of the number of trees planted per acre,x,if x x15 30.

答案

每亩种30棵树时,每棵树产150个,每多种一棵树则每棵树减产3个,试将一亩地的总产量Y表示成该地所种树X棵的函数,其中X>=30
y=[150-3*(x-30)]*x=-3x^2+240x