Optimal point on budget line

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Optimal point on budget line발음듣기

So let's just review what we've seen with budget lines.발음듣기

Let's say I'm making $20 a month.발음듣기

So my income is $20 per month.발음듣기

The price of chocolate is one dollar per bar.발음듣기

And at the price of fruit is two dollars per pound.발음듣기

And we've already done this before but I'll just redraw a budget line.발음듣기

So this axis, let's say this is the quantity of chocolate.발음듣기

I could've picked it either way.발음듣기

And that is the quantity of fruit.발음듣기

Not quantity of four. Quantity of fruit.발음듣기

If I spent all my money on chocolate I could buy 20 bars of chocolate a month.발음듣기

So that is 20. This is 10 right over here.발음듣기

At these prices if I spent all of my money on fruit I could buy 10 lbs. per month.발음듣기

So this is 10. That's 10 pounds per month.발음듣기

So I have a budget line that looks like this. A budget line that looks like this.발음듣기

And the equation of this budget line is going to be - well, I could write it like this.발음듣기

My budget (20) is going to be equal to the price of chocolate, which is 1 times the quantity of chocolate.발음듣기

So this is 1 times the quantity of chocolate.발음듣기

Plus the price of fruit, which is 2, times the quantity of fruit.발음듣기

And if want to write this explicitly in terms of my quantity of chocolate, since I put that on my vertical axis, since that tends to be the more dependent axis, I can just subtract 2 times the quantity of fruit from both sides.발음듣기

And I can flip them and get my quantity of chocolate is equal to 20 minus 2 times my quantity of fruit.발음듣기

And I get this budget line right over there.발음듣기

We've also looked at the idea of an indifference curve.발음듣기

So, for example, let's say I'm sitting at some point on my budget line where I have - let's say I am consuming 18 bars of chocolate and one pound of fruit.발음듣기

18. And you can verify that and it makes sense.발음듣기

It's going to be $18 plus 2 is $20.발음듣기

So let's say I'm at this point on my budget line. 18 bars of chocolate - so this is in bars - and one pound of fruit per month.발음듣기

That is 1 and this is in pounds.발음듣기

And this is chocolate. And this is fruit right over here.발음듣기

Well, we know we have this idea of an indifference curve;발음듣기

there's different combinations of chocolate and fruit to which we are indifferent, to which we would get the same exact total utility.발음듣기

And so we can plot all those points, I'll do it in white, it could look something like this.발음듣기

I'll do it as a dotted line.발음듣기

It makes it a little bit easier. I should draw it like this.발음듣기

So let's say I'm indifferent between any of these points - those points over there.발음듣기

Let me draw it a little bit better.발음듣기

So between any of these points right over there.발음듣기

So, for example, I could have 18 bars of chocolate and one pound of fruit, or I could have - let's say that is four bars of chocolate and, let's see, and roughly eight pounds of fruit. I'm indifferent.발음듣기

I get the same exact total utility.발음듣기

Now, am I maximizing my total utility at either of those points?발음듣기

Well, we've already seen that anything to the top right of our indifference curve, of this white curve over here Let me label this.발음듣기

This is our indifference curve. Everything to the top right of our indifference curve is preferable.발음듣기

We're going to get more total utility.발음듣기

Let me color that in. So everything to the top right of our indifference curve is going to be preferable.발음듣기

So all of these other points on our budget line, even a few points below our budget line, where we would actually save money, are preferable.발음듣기

So either of these points are not going to maximize our total utility.발음듣기

We can maximize our total utility at all these other points in between along our budget line.발음듣기

So to actually maximize out total utility, what we want to do is find a point on our budget line that is just tangent, that just touches at exactly one point one of our indifference curves.발음듣기

We can have an infinite number of indifference curves.발음듣기

There could be one that looks like that.발음듣기

There could be another indifference curve that looks like that.발음듣기

All that says is that we are indifferent between any points on this curve.발음듣기

And so there is an indifference curve that touches exactly this budget line, or exactly touches the line at one point.발음듣기

So I might have an indifference curve that looks like this.발음듣기

Let's do this in a vibrant color, in magenta.발음듣기

So I could have an indifference curve that looks like this.발음듣기

And because it's tangent - it touches in exactly one point - and also the slope of my indifference curve which we've learned is the marginal rate of substitution, is the exact same as the slope of our budget line right over there, which we learned earlier was the relative price.발음듣기

So this right here is the optimal allocation on our budget line.발음듣기

That right here is optimal. And how do we know it is optimal?발음듣기

Well, there's no other point on the budget line that is to the top right.발음듣기

In fact, every other point on our budget line is to the bottom left of this indifference curve.발음듣기

So every other point on our budget line is NOT preferable.발음듣기

So remember, everything below an indifference curve, all this shaded area - we should actually do it in another color.발음듣기

Because indifference curve we are indifferent: but everything below an indifference curve - so all of this area in green - is not preferable.발음듣기

And every other point on the budget line is not preferable to that point right over there.발음듣기

Because that is the only point - or I guess you could say every other point on our budget line is NOT preferable to the points on the indifference curve.발음듣기

So they are also not preferable to that point right over there, which actually is on the indifference curve.발음듣기

Now let's think about what happens if the price of fruit were to go down.발음듣기

So, the price of fruit were to go from $2 to $1. $2 to $1 per pound.발음듣기

So if the price went from 2 to 1 dollar, then our actual budget line will look different.발음듣기

Our new budget line - I'll do it in blue - would look like this.발음듣기

If we spent all our money on chocolate we could buy 20 bars.발음듣기

If we spent all of our money on fruit at the new price, we could buy 20 pounds of fruit.발음듣기

So our new budget line would look something like that.발음듣기

Would look something like that. So that is our new budget line.발음듣기

New budget line. So now what would be the optimal allocation of our dollars? Or, the best combination that we would buy?발음듣기

Well, we would do the exact same exercise.발음듣기

Assuming we had data on all of these indifference curves, we would find the indifference curve that is exactly tangent to our new budget line.발음듣기

So let's say that this point right over here is exactly tangent to another indifference curve.발음듣기

So just like that. There's another indifference curve that looks like that.발음듣기

Try and make it a little bit neater, so it looks something like that.발음듣기

And so based on how the price - assume we have access to these many, many indifference curves we can now see based on how a price, with all else equal, how a change in the price of fruit changed the quantity of fruit we demanded.발음듣기

Because now our optimal spent is this point on our new budget line, which looks like it's about, give or take, about 10 pounds of fruit.발음듣기

So all of a sudden, when we were - let's think about just the fruit.발음듣기

Everything else we're holding equal. So just the fruit.발음듣기

When the price was two, the quantity demanded was eight pounds.발음듣기

And now when the price is one, the quantity demanded is 10 pounds.발음듣기

So what we're actually doing - and once again we're kind of looking at the exact same ideas from different directions.발음듣기

Before we looked at it in terms of marginal utility per dollar and we thought about how you maximize it.발음듣기

And we were able to change the prices and then derive a demand curve from that.발음듣기

Here we're just looking at it from a slightly different lens.발음듣기

But they really are all of the same ideas.발음듣기

But assuming we had access to a bunch of indifference curves, we can see how a change in price changes our budget line, and how that would change the optimal quantity we would want of a given product.발음듣기

So, for example we could keep doing this and we could plot our new demand curve.발음듣기

So I could do a demand curve now for fruit.발음듣기

At least I have two points on that demand curve.발음듣기

So if this is the price of fruit, and this is the quantity demanded of fruit, when the price is two the quantity is eight.발음듣기

And when the price is - actually let me do it a little bit different.발음듣기

When the price is two, and these aren't to scale, the quantity demanded is eight.발음듣기

And then - actually I should do it here.발음듣기

And these aren't to scale. When the price is one the quantity demanded is 10. Two, eight, quantity demanded is 10.발음듣기

And so our demand curve - these are two points on it - we could keep changing it up.발음듣기

Assuming we had access to a bunch of indifference curves, we could keep changing it up and eventually plot our demand curve, that might look something like that.발음듣기

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