Sum of Co-ordinates | AMC-10A, 2014 | Problem 21

Positive integers $a$ and $b$ are such that the graphs of $y=ax+5$ and $y=3x+b$ intersect the $x$-axis at the same point. What is the sum of all possible $x$-coordinates of these points of intersection?

The given equations are $y=ax+5$\(\Rightarrow x=\frac{-5}{a}\).....(1)

$y=3x+b$\(\Rightarrow x=\frac{-b}{3}\)............(2)

Since two lines intersect the $x$-axis at the same point,then at first we have to find out the common point on x -axis.........

Now the intercept form of the given two equations will be

\(\frac{x}{(-5/a)} +\frac{y}{5}=1\) ,Therefore the straight line intersect x-axis at the point (\(\frac{-5}{a},0\))

\(\frac{x}{(-b/3)}+\frac{y}{b}=1\) Therefore the straight line intersect x-axis at the point (\(\frac{-b}{3},0)\).

Can you now finish the problem ..........

Since two lines intersect the $x$-axis at the same point so we may say that

(\(\frac{-5}{a},0\))=(\(\frac{-b}{3},0)\)

\(\Rightarrow ab=15\)

The only possible pair (a,b) will be \((1,15),(3,5),(5,3),(15,1)\)

can you finish the problem........

Now if we put the values \((1,15),(3,5),(5,3),(15,1)\) in (1) & (2) we will get \(-5\),\(\frac{-5}{3}\),\(-1\),\(\frac{1}{3}\)

Therefore the sun will be \(-8\)

AMC - AIME Boot Camp... for brilliant students.

Use our exclusive one-on-one plus group class system to prepare for Math Olympiad

Learn More

Subscribe to Cheenta at Youtube