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I want to simplify an equation in terms of $x$. And I tried the Simplify and Collect functions by assuming $x$ is positive real. But the result does not collect the $x$ together.

a = x^4;
b = (y/x^(3/8))^(7/9);
Assuming[{x > 0, x ∈ Reals}, Simplify[a b, x]]
Collect[a b, x] //. Assuming -> x > 0

$\text{True}^4 \left(\frac{y}{\text{True}^{3/8}}\right)^{7/9}$

$x^4 \left(\frac{y}{x^{3/8}}\right)^{7/9}$

Does anyone know how to do it correctly?

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    $\begingroup$ % // PowerExpand $\endgroup$ – zhk Feb 2 '17 at 17:59
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The appropriate functionality to use here is PowerExpand.

a = x^4;
b = (y/x^(3/8))^(7/9);
a b // PowerExpand

x^(89/24) y^(7/9)

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Simplify has wrong syntax: Simplify[expression, conditions]

Then:

Simplify[a b, {x > 0}]

x^(89/24) y^(7/9)

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