DELTA_xi = @(x) besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(1, x)*bessely(0, x) - besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(0, x)*bessely(1, x) - besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(0, x)*besselk(1, x) + besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(1, x)*besselk(0, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(0, x)*bessely(1, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(1, x)*bessely(0, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(0, x)*besselj(1, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(1, x)*besselj(0, x)
fzero(DELTA_xi, 3.5)

DELTA_xi = @(x) besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(1, x)*bessely(0, x) - besseli(1, (3*x)/10)*besselj(1, (3*x)/10)*besselk(0, x)*bessely(1, x) - besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(0, x)*besselk(1, x) + besseli(1, (3*x)/10)*bessely(1, (3*x)/10)*besselj(1, x)*besselk(0, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(0, x)*bessely(1, x) - besselj(1, (3*x)/10)*besselk(1, (3*x)/10)*besseli(1, x)*bessely(0, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(0, x)*besselj(1, x) + besselk(1, (3*x)/10)*bessely(1, (3*x)/10)*besseli(1, x)*besselj(0, x)
fzero(DELTA_xi, 3.5)

3.8173

fzero(@(x)eval(DELTA_xi), 3.5)

fcn1 = @(x) besselj(0,x);

fcn2 = @(x) DELTA_xi;

fzero(@(x) subs(DELTA_xi,'x',x), 3.5)

fzero(@(y) subs(DELTA_xi,'x',y), 3.5)

fzero(@(y) eval(DELTA_xi), 3.5)