題目
11. Let
A be the region bounded above by the graph of
y=sin(πx) and below by the graph of
y=x3−4x.
(A) (2.1,0.8) falls into the region A.
(B) The area of A can be represented by ∫02[sin(πx)−(x3−4x)]dx.
(C) The area of A is equal to 4.
(D) The region A is the base of a solid. For this solid, each cross section perpendicular to the x-axis is a square. The volume of this solid is equal to ∫02[sin(πx)−(x3−4x)]2dx.
(E) The region A models the surface of a small pond. At all points in A at a distance x from the y-axis, the depth of the water is given by h(x)=3−x. The volume of water in the pond is equal to ∫02(3−x)[sin(πx)−(x3−4x)]dx.
解答
解法一
思路
展開
- 兩圖在 x=0,2 相交,且中間正是上減下;逐項依區域、面積、截面體積與密度體積判斷。
答題過程
展開
在 0≤x≤2,上方曲線是 sin(πx)。故 (B) 的面積式正確,且
∫02[sin(πx)−x3+4x],dx=4,
所以 (C) 正確。平方截面體積為高度平方的積分,(D) 正確;以 3−x 作深度的水量積分亦正確。點 (2.1,0.8) 不在區域內。答案為 (B)、(C)、(D)、(E)。