# Laplace transform in a block system

I'm trying to understand the resolution of the problem 5.11, but I can't understand how she gets the expression in the rectangle: $$\dot{x}_1=-x_1+2u$$ How can I get the expression in the rectangle?

• I don't understand why someone would write the small case s so confusingly then almost do the same when writing an x. – Andy aka Jan 7 at 16:11

$$\ u \$$ is the input signal it might be a real number so $$\ \dot{u}=0 \$$
so i would expect the result to be : $$\ \dot{x}_1=−x_1+3u \$$
• Why did she obtain two different expressions for $\dot{x_1}$? What is the correct expression? – Carmen González Jan 7 at 16:26
• I put the rest of the resolution in the post. I don't understand why she gets two expressions for $\dot{x_1}$. – Carmen González Jan 7 at 22:32