GNU Octave Equation Help

[Mike]

Hi, I am using GNU Octave to solve a numerical integration but I keep getting
Parse and Syntax Error for the equation that I write. For simplicity, I
wrote a simpler equation to help demonstrate my problem. The equation
written is part of the formula used for the surface area of an object via
volume of revolution. The error message is inserted as a picture and I've
also written the equation below:
y=(sqrt(25-x.^2))*(sqrt(1+((1/2)*(-2x)*(1/(sqrt(25-x.^2)))).^2)) 
An error keeps coming up on the "x" part of the "(-2x)" in the middle of the
equation.   If you can find the mistake or let me know an alternative
method, please let me know ASAP! Thanks!

Error Message: 
<http://octave.1599824.n4.nabble.com/file/n4672237/Capture.png> 



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