# How to simplify an expression by collecting like terms? [closed]

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?

## closed as off-topic by march, MarcoB, m_goldberg, corey979, C. E.Feb 3 '17 at 6:26

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• "This question arises due to a simple mistake such as a trivial syntax error, incorrect capitalization, spelling mistake, or other typographical error and is unlikely to help any future visitors, or else it is easily found in the documentation." – march, MarcoB, m_goldberg, corey979, C. E.
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• % // PowerExpand – zhk Feb 2 '17 at 17:59

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)

Simplify has wrong syntax: Simplify[expression, conditions]

Then:

Simplify[a b, {x > 0}]


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