find coordinates for x and y intercept

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Hi Krissy,

The answer by Vivian L. correctly describes how to find the y-intercept. Now, let's consider how to find the x-intercept.

The x-intercept is where the graph crosses the X-axis. At this point, y=0.

So, we plug y=0 into the equation above:

-5x + 6(0) = 100

-5x + 0 = 100

x = -20 (dividing both sides by -5).

So, the x-intercept is the point (-20,0).

Vivian got off to a great start, but didn't explain any methods for finding the x-intercept.

One way is to manipulate the equation from y = m x + b

to a new form y = m ( x + b/m) by factoring m out of both terms.

For the students who know that

a) the graph of a function f(x-a) is the same as the graph of f(x) except that the graph of f(x-a) will have been translated a units to the right, and

b) the graph of y = mx (where b is zero) always goes through the origin,

this is a great way to apply that knowledge and read-off the x-intercept in a fool-proof way.

If you know these two premises are true, you can see that once you have y=mx+b, the x-intercept will be the graph of y = mx shifted m/b to the right, so the x-intercept that was formerly at the origin will now be at m/b.

For those who find all of that too confusing, think of it this way:

Slope = rise / run

Therefore, if we solve that equation for run, we get Run = rise / slope

Therefore, if our y-intercept is b, what run will give us a rise of -b from the y-intercept (which is 0,b)?

Plug (-b) into our new formula for "Run":

Run = (-b)/m.

A run of -b/m means we need to travel horizontally -b/m from the x-position of the y-intercept (0), to reach the x-axis to accumulate a total rise of -b. Traveling -b/m from 0 puts at the x-coordinate of -b/m.

To check this result, use it in the equation.

What will be the value of y if x = -b /m ?

y = m x + b

y= m (-b/m) + b

But m * (-b/m) is just m(-b/m) (multiplying by m and dividing by m cancels out and has no effect on -b)

y = -b + b

y = 0

Therefore, yes, computing -b/m after finding the equation y = mx+b always gives an x-intercept.

So go ahead and figure out the x-intercept.

Hi Krissy;

-5x+6y=100

The standard formula to find x the y-intercept when x=0, is...

y=mx+b

b represents the y-intercept.

So...

-5x+6y=100

We must isolate y...

Let's add 5x to both sides...

5x+-5x+6y=100+5x

6y=5x+100

Let's divide both sides by 6...

6y/6=(5/6)x+100/6

y=(5/6)x+50/3

The slope is 5/6

The y-intercept is 50/3

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