Why when we try to represent a sinus function we use a cosinus expression some times and cosinus another times like the case in matlab?

we asked in an exercise to create a sinusoidal signal of 0.8 seconds with an amplitude of 1 and frequency equal to 100 Hz sampled to 1000 Hz , but in the answer i found:

    A=1;fs=100;fe=1000;
    te=1/fe;
    t=0:te:0.8;
    s=A*cos(2*pi*fs*t);
    plot(s);

And in another exercise: write a script for a sinusoidal signal amplitude 1 and 200KHz frequency, over a period N = 64, sampled at 150 KHz.the answer was :

%Signal

fs=200;N=64;

ts=1/fs;

A=1;

subplot(211);

t=0:0.00001:N*ts;

x=sin(2*pi*t*fs);

plot(t,x);

title('signal sinusoidal');

%Echantillonnage

Fe=input('Fe=');

te=1/Fe;

t1=0:te:N*te;

xe=sin(2*pi*t1*fs);

subplot(212);

stem(t1,xe);

I'm so confused ,when to put a sin and when to put a cosin ;My question is that why they puted a cosin instead of a sin is there is some kind of rule behind it ; and thanks in advance.

Answer 1

[Jos (10584)]

Both a cosine and a sine are sinusoids, but one is shifted in phase compared to the other

cos(x) = sin(x + (pi/2))