# Partial derivatives economics question

#### bart11

##### New member
If a and b are constants, compute the expression KY'(K) + LY'(L) for Y = AK^a + BL^a

Y'(K) means partial derivative with respect to K by the way. The answer in the book is KY'(K) + LY'(L) = aY

I'm not sure what they did or what they're asking :/

#### Krizalid

##### Active member
It's pretty straightforward. I'll do the first one for you. We have $y_k=Aak^{a-1}$ (because the other term doesn't have a $k$ since it's the partial derivative), and $y_k$ denotes the partial derivative respecto to $k.$

Try the other one and substitute.

#### bart11

##### New member
It's pretty straightforward. I'll do the first one for you. We have $y_k=Aak^{a-1}$ (because the other term doesn't have a $k$ since it's the partial derivative), and $y_k$ denotes the partial derivative respecto to $k.$

Try the other one and substitute.
Thanks! I get it now, was confused on whether the A and B were variables or not. Thanks!