2. 外语
  3. A flat rectangular picture, represented by the unshaded region in the figure above, is mounted in a flat rectangular frame, repr

A flat rectangular picture, represented by the unshaded region in the figure above, is mounted in a flat rectangular frame, repr

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问题
A flat rectangular picture, represented by the unshaded region in the figure above, is mounted in a flat rectangular frame, represented by the shaded region. The frame is 1 inch wide on all sides. For what value of x, in inches, is the area of the frame equal to the area of the picture?
选项 A、4
B、5
C、6
D、7
E、8
答案C
解析 In this question, you are asked to determine the value of x for which the area of the frame is equal to the area of the picture. To do this, you need to express both the area of the frame and the area of the picture in terms of x and then find the value of x for which the two expressions are equal.
    The area of the picture is the area of the inner rectangle, and the area of the frame is the area of the outer rectangle minus the area of the inner rectangle. Since the area of a rectangle is the length times the width, you need to know the length and width of the inner and outer rectangles.
    In the figure, you are given that the length of the inner rectangle is x inches, but the width is not given. However, since you know that the width of the frame is 1 inch, it follows that the width of the inner rectangle is equal to the width of the outer rectangle minus 2 inches, or x - 2 inches. Thus, the area of the inner rectangle is x(x — 2)square inches.
    In the figure, you are given that the width of the outer rectangle is x inches, but the length is not given. However, since you know that the width of the frame is 1 inch, it follows that the length of the outer rectangle is equal to the length of the inner rectangle plus 2 inches, or x + 2 inches. Thus, the area of the outer rectangle is x(x + 2)square inches.
    Since the area of the frame is the area of the outer rectangle minus the area of the inner rectangle, the area of the frame is x(x + 2)- x(x - 2)= x2 + 2x - x2 + 2x = 4x square inches.
    Now you are ready to set up the equation. Set the expression for the area of the picture equal to the expression for the area of the frame and solve the resulting equation for x, as follows.
x(x - 2)= 4x
x2 - 2x = 4x
x2 - 6x = 0
x(x - 6)= 0
There are two solutions to the equation, x = 0 and x = 6. Since x represents the length of a picture, in inches, the solution x = 0 does not make sense in this context. Therefore, when x = 6, the area of the picture equals the area of the frame. The correct answer is Choice C.
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