Why is the total revenue curve of a price-taking firm an upward-sloping straight line? Why does the curve pass

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Q. Why is the total revenue curve of a price-taking firm an upward-sloping straight line? Why does the curve pass through the origin?

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Concept understanding — Marginal Revenue Constant

Marginal Revenue Constant: When Every Extra Unit Earns the Same

Think about a lemonade stand. On a hot day, you sell the first glass for ₹20. The second glass? Still ₹20. The third? ₹20 again. Every time you sell one more glass, your total revenue goes up by exactly ₹20. That ₹20 is your marginal revenue — and it stays the same no matter how many glasses you sell.

That is the simplest case of constant marginal revenue. But in economics, this situation is special, not automatic. Let's see why.

The Precise Meaning

Marginal Revenue (MR) is the change in total revenue when you sell one additional unit of output.

MR = \frac{\Delta TR}{\Delta Q}

Where:

$\Delta TR$ = change in total revenue
$\Delta Q$ = change in quantity sold (usually 1 unit)

When MR is constant, it means every extra unit you sell adds exactly the same amount to your total revenue. The number does not change as you sell more.

When Does MR Stay Constant?

This happens only when the firm can sell any quantity at the same price — that is, when the firm is a price taker. In perfect competition, the market sets the price, and the individual firm cannot influence it. So:

MR = P

The marginal revenue from selling one more unit is simply the market price. And since the price does not change with the firm's output, MR is constant.

Important

In perfect competition, $MR = AR = P$ at every level of output. The MR curve is a horizontal straight line at the market price.

Why It Matters

Constant MR changes how a firm thinks about profit. If MR is constant, the firm's total revenue is a straight line:

TR = P \times Q

Every extra unit adds exactly $P$ to revenue. So the firm's decision to produce more depends entirely on whether the marginal cost of that unit is less than or greater than this constant MR.

Note

If MR is constant but MC is rising (as it usually does), the firm will keep producing until $MR = MC$ . That is the profit-maximising output.

The Contrast: When MR Is Not Constant

Most real-world firms face downward-sloping demand. To sell more, they must lower the price on all units. So the extra revenue from selling one more unit is less than the price — MR falls as output rises. That is the world of monopoly, monopolistic competition, and oligopoly. …

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For a price-taking firm, price is constant, so total revenue ( $TR = P \times Q$ ) is a linear function of output. It passes through the origin because zero output yields zero revenue.

A price-taking firm operates in a perfectly competitive market. The defining feature of such a market is that the firm is so small relative to the entire market that its own output decisions cannot influence the market price. The price is determined by the intersection of market demand and market supply, and the firm simply accepts this price as given — it is a "price taker."

Because the price $P$ is constant for every unit the firm sells, the total revenue function is:

TR = P \times Q

Here, $P$ is a fixed positive number. This is a linear equation of the form $y = mx$ , where the slope $m = P$ and the intercept is zero. So as output $Q$ increases, total revenue increases proportionally. Every additional unit sold adds exactly the same amount — the market price — to total revenue. That is why the total revenue curve is an upward-sloping straight line.

TR = P \times Q \quad \text{where } P \text{ is constant}

Now, why does this curve pass through the origin? The logic is straightforward: if the firm produces and sells nothing ( $Q = 0$ ), it earns no revenue at all. There are no fixed revenues or subsidies assumed here. So $TR = P \times 0 = 0$ . The point $(0,0)$ lies on the curve, meaning the curve starts at the origin. …

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Q.Why is the total revenue curve of a price-taking firm an upward-sloping straight line? Why does the curve pass through the origin?

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