Parallelogram on the Coordinate Plane

Geometry Level 4

A parallelogram on the coordinate plane is defined by the following lines: { y = 4 x + 5 y = 4 x + 3 y = n x + 5 y = n x + 3 \left\{\begin{array}{l}y=4x+5 \\ y=4x+3 \\ y=nx+5\\ y=nx+3\end{array}\right.

If the area of the parallelogram is 16 16 , then n n can be expressed as p q \dfrac{p}{q} , where p , q p,q are coprime positive integers. Find the smallest possible value of p + q p+q .


The answer is 19.

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2 solutions

Dinesh Reddy
Feb 27, 2014

Lets Start with basic area equation of a parallelogram
Area=height*base;
Now see the line are given as directly parallel pairs
so we can find height(perpendicular distance between lines){y=4x+5,y=4x+3} is 2/17^(1/2)--------(1) Now we got height lets look in to base calculation....
Intersection between lines y=4x+5,y=nx+5 On solving we get (0,5)
Intersection between lines y=4x+3,y=nx+5 On solving we get(2/4-n,20-3n/4-n)
Find distance between these two points...on Calculating we get 68^(1/2)/|4-n| --------------------(2) By multiplying 1,2 we get area of parallelogram which is equal to 16(given)
By equating both and on solving we get n=15/4 or 17/4


i put in 21 at the beginning.....

Anshuman Karthik - 7 years, 3 months ago

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did you get credit? I found both and guessed that 19 might be right. So, idk if 21 would have worked.

Skylar Saveland - 7 years, 3 months ago

me too XD

Jerome Christian Lumacad - 7 years, 2 months ago
Heitor Farias
Feb 26, 2014

Hey, this question has 2 solutions! n=17/4 and n'=15/4.

Sorry, forgot that there were two solutions. Wording edited.

Daniel Liu - 7 years, 2 months ago

they have asked smallest value which is 15+4

Hardik Aggarwal - 7 years, 1 month ago

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