error while trying to plot four sinusoids

script code

t= -5:0.1:5;

x1(t)= 5*cos(2*pi*(15)*t+0.5*pi);                                                                                       
x2(t)= 5*cos(2*pi*(15)*t+0.25*pi);
x3(t)= 5*cos(2*pi*(15)*t+0.4*pi);
x4(t)= 5*cos(2*pi*(15)*t+0.9*pi);
plot (t, x1(t), x2(t),x3(t),x4(t))
title ('Plot Of Four Sinusoids')

error

??? Subscript indices must either be real positive integers or logicals.

Error in ==> PlotOfFourSinusoids at 2 x1(t)= 5*cos(2*pi*(15)*t+0.5*pi);