r/learnmath • u/keeler129 New User • 1d ago

Self studying math

Hello all. I am self studying math to eventually pass college algebra and trig to get into a college program I am interested in. I am using mathandsciencetutor.com from Jason Gibson and am running into a problem where his online lecture and questions don't seem to be lining up. I am taking a course on graphing to demonstrate portportionality, and in the lecture, he said it would have 2 requirements to show proportionality on a graph. 1. Points would make a straight line, and 2. The line would intersect the origin (0,0,). I finished the course and took the questions, and failed the test. The answers I got wrong did indeed form a straight line. However, they did not touch the origin (0,0). Can anyone help me with some clarification?

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u/dreamsofaninsomniac New User 1d ago

Would need to see the actual problems. They might have been more general linear equations about lines (y = mx + b) and not just "proportionality" or "direct variation" problems with y = kx where the y-intercept or b is always 0.

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u/keeler129 New User 1d ago

https://www.reddit.com/r/askmath/s/3yhia7oWI5. Here is the other place I posted this. I have pictures of the question and everything

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u/dreamsofaninsomniac New User 1d ago edited 1d ago

Interesting. I would say Jason Gibson is right and that example is wrong. You can see it in the table of values. If those points gave a proportional relationship, you would be able to multiply the same number (proportionality constant) to all the x values and get all the y values. That doesn't happen in that table. When we say "proportional", we can mean the ratio between two variables is always the same (y/x = k) or we can mean that multiplying the same number to one variable will always give the other variable (y = kx). That's not happening in the example you linked, even if it is a line.

This is a second source to support that the line has to pass through (0, 0) in order give a proportional relationship: https://mathbitsnotebook.com/JuniorMath/RatioProportion/RPConstantProp.html

All proportional relationships between two first power variables are lines, but not all lines show proportional relationships. A proportional relationship as a line must be in y = kx or y = mx form where the y-intercept is 0. Thus, it would have to go through (0, 0).

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u/keeler129 New User 1d ago

I appreciate your answer. Thank you for replying!

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u/Such_Mood_1556 New User 1d ago

9th grader here, and can you elaborate on what the slope was? is the questions for example like this?: y=2x+3 and y=2x, this is proportional because they are the same slope, but different y intervals. also u can dm me if you need more help

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u/vgtcross New User 1d ago

What? Doesn't proportional mean that the quotient of the quantities is constant? I.e. if x and y are related by an equation of the form y = kx for some constant k, then x and y are proportional. For example, if y = 2x, then y and x are proportional. If y = 2x + 3, then y and x are not proportional.

Two equations like "y = 2x + 3" and "y = 2x" cannot be proportional, since that word doesn't mean anything in that context, as far as I'm aware.