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A line which has one end, but stretches off to infinity is called a ray. If we want to show an interval such as [2,infinity) which means all the numbers between two an infinity including two, we put a close dot at two and draw a ray going to the right. Similarly for (negative infinity, 2] we draw the closed dot at 2 but this time the ray goes to the left. Now if we draw an open circle, or a the out line of a dot this mean the interval (2,infinity) where we are considering all the numbers very close to two but not two itself This is an open half line and it represents an open interval.

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15y ago

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