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In security printing, void pantograph refers to a method of making copy-evident and tamper-resistant patterns in the background of a document. Normally these are invisible to the eye, but become obvious when the document is photocopied. Typically they spell out "void", "copy", "invalid" or some other indicator message. [1]
y=f(x)=.5x+1 or f(x,y)=x-2y+2=0 Positive and negative half-planes. The slope-intercept form of a line is written as = = + where is the slope and is the y-intercept. Because this is a function of only , it can't represent a vertical line.