17. Let the operation \[ \Uparrow \] be defined by \[a \Uparrow b = \frac{a + b}{a - b}\] for all numbers \[a\] and \[b\], where \[a # b\]. If \[1 \Uparrow 2 = 2 \Uparrow x\], what is the value of \[x\]?
(A) 4
(B) 3
(C) 2
(D) 1
(E) 0

18. During a sale, a customer can buy one shirt for \[x\] dollars. Each additional shirt the customer buys costs \[z\] dollars less than the first shirt. For example, the cost of the second shirt is \[x - z\] dollars. Which of the following represents the customer's cost, in dollars, for \[n\] shirts bought during this sale?
(A) \[x + (n - 1)(x - z)\]
(B) \[x + n(x - z)\]
(C) \[n(x - z)\]
(D) \[\frac{x + (x - z)}{n}\]
(E) \[(x - z) + \frac{(x - z)}{n}\]
p2sec1.19
19. In the figure above, \[QR\] is the arc of a circle with center \[P\]. If the length of arc \[QR\] is \[6\pi \], what is the area of sector \[PQR\]?
(A) \[108\pi \]
(B) \[72\pi \]
(C) \[54\pi \]
(D) \[36\pi \]
(E) \[9\pi\]

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