Possible number of lines

Geometry Level 4

Suppose that the x x -intercept of a line is a (positive) prime number and its y y -intercept is a positive integer. How many such lines exist that pass through the point ( 4 , 3 ) ? (4, 3)?

1 2 0 3

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2 solutions

Harrison Wang
Jul 23, 2014

Let the x-intercept of the line be ( x 1 , 0 ) (x_1, 0) , and the y-intercept be ( 0 , y 1 ) (0, y_1) . Then the slope of the line is y 1 x 1 -\frac{y_1}{x_1} and the equation y y 1 = y 1 x 1 x y - y_1 = -\frac{y_1}{x_1} x . We can substitute the point (4, 3) into this equation to get 3 = 4 y 1 x 1 + y 1 3 = -\frac{4y_1}{x_1} + y_1 . The rest is nonrigorous casework. Starting from observation when x 1 = 2 , 3 , 5 x_1 = 2, 3, 5 , etc. we see that the sets ( x 1 , y 1 ) (x_1, y_1) that allow for such line include (5, 15) and (7, 7).

Edmund Letaba
Jul 23, 2014

only x intercept y=3 and the y intercept x=4. will pass through the point (4,3)

nice one. it's like cross only

joram otero - 6 years, 10 months ago

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