# Math Help - parametric arc length

1. ## parametric arc length

Consider the parametric equation
x=4(cos+sin)
y=4(sincos)

What is the length of the curve for =0 to =9pi/10

the formula i believe is

L=integral from t0 to t1 sqrt((dx/dt)^2+(dy/dt)^2)dt

ive tried pluggin in the derivatives of of x and y for dx dt and integrating from 0 to 9pi/10 but its not working

2. I think that is the correct formula. What is the problem? I get $x'(\theta) = 4\,\theta\,\cos(\theta), ~y'(\theta) = 4\,\theta\,\sin(\theta)$. Gives an arc length of $\frac{81 \pi^2}{50}$, which looks plausible.