帮我解释一下matlab中这两句是什么意思？

clearN=12;t=(0:0.02/N:0.02)';这个括号后为什么有一撇？？m=size(t);y=sin(2*pi*50*t);y1=[0    y(1:m-1)];这一句神马意思？？ym=sqrt((y.^2+y1.^2-2*y.*y1*cos(2*pi/N))/(sin(2*pi/N))^2);subplot(221);plot(t,y,'-o',t,y1,'-o');legend('y(k)','y(k-1)');title('liangcaiyangzhisuanfa');subplot(223);plot(t,ym,'-o');axis([0,0.02,0,1.2]);xlabel('t/s');ylabel('ym');text(0.01,0.6,'N=12');