(Section 1.8: x=fy()) 1.8.4 PART D: THE HORIZONTAL LINE TEST (HLT)In Section 1.2, we said that an equation in xand ydescribes yas a function of x±its graph passes the Vertical Line Test (VLT)in the xy-plane. We now have: The Horizontal Line Test (HLT)A curve in a coordinate plane passes the Horizontal Line Test (HLT)±There is nohorizontal line that intersects the curve more than once. An equationin xand ydescribes xas a function of y±Its graphin the xy-plane passes the HLT. • Then, there is noinput ythat yields more than one output x. • Then, we can write x=, wherefis a function. • A “curve” could be a straight line.
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