Find Price elasticity of demand in this example ln(Q) = 10-0.7ln(P), where P is price and Q is demand.

What is the price elasticity of demand?
Did you know you can write price elasticity $\varepsilon$ as $\varepsilon \equiv  \frac{d \log Q}{d \log P}$? 

(Indeed, $\frac{d \log Q}{d \log P} =\frac{dQ}{dP} \frac{P}{Q}$. Check out wiki on elasticity of a function if you want more detail!) 

Using this "log notation" for elasticity, it is easy to see without putting pen to paper that in this example, $\varepsilon = -0.7$.