Wrong answer in below code?

bw = 36;

angle = 60;

syms ds

tw = bw + 2*(ds / tand(angle));

nm = 0.025;

Area = 0.5*(bw + tw)*ds;

P = bw + 2*(ds/sind(angle));

S = 0.00001;

A = (((Area)^(5/3))/((P)^(2/3))) == ((200*nm)/(S^0.5))

height =(vpasolve(A,ds));

OUTPUT: - 49.95106 - 10.88106i

The output is wrong, the correct answer for the above code should be 10.19

Can anyone please tell me why I am not getting the actual answer as 10.19

Answer 1

[Walter Roberson]

height = vpasolve(A, ds, 1);

[Raj Arora]

Thanks walter can you please tell me what initial paramter here means

[Walter Roberson]

The 1 here is the initial guess. vpasolve uses a Newton Raphson type method, so the results can depend on the initial location when there are multiple roots. The output you saw was not an error: it was just a different root. Your formula includes values to the (5/3) power, so if the initial guess takes the term negative, then negative to a fractional power is complex valued.

[G A]

Walter, why my (not correct) answer (I have deleted it already)

height = vpasolve(A,nm);

does give the right result?

[Walter Roberson]

Most any positive initial value should work

[G A]

Thanks, Walter - I understood postfactum that nm was interpreted as the initial value because it cannot be an argument.