# Price elasticity of Demand question watch

1. Hey!

Anyway, I’m still a little confused with why price elasticity of demand varies at every level in a straight line demand curve. “It is wrong to say that price elasticity is constant”. BUT WHY? Mathematically, PED is %change in q/ % change in P, which means it is replica of dy/dx, which calculates the gradient. But the gradient does not change when it is in a straight line.

I just find it a little weird. I know its related to the relationship between price elasticity and marginal revenue graph... but still.
Hey!

Anyway, I’m still a little confused with why price elasticity of demand varies at every level in a straight line demand curve. “It is wrong to say that price elasticity is constant”. BUT WHY? Mathematically, PED is %change in q/ % change in P, which means it is replica of dy/dx, which calculates the gradient. But the gradient does not change when it is in a straight line.

I just find it a little weird. I know its related to the relationship between price elasticity and marginal revenue graph... but still.
Not quite, I'm pretty sure everyone studying economics has come across the same thought though =D

Right, so starting from your (correct) statement that PeD = % change in Q / % change in P :

Thus:

Now, while it is true that [delta Q/delta P] is constant, it's the first fraction which changes across the demand curve - P and Q vary. The mistake in your logic is that dy/dx is not a percentage change.

Hope this helps.

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