Find the arc length of y=20cosh(x/20) from x=-20 to x=20?

Answer 1

It's the integral from -20 to 20 of sqrt(1 + y'^2) dx

where here

y' = sinh(x/20), so 1 + y'^2 = 1 + sinh^2 (x/20) = cosh^2 (x/20)