x = linspace(-2*pi,2*pi,1000);
y = sin(x);
n = 300;
x0 = x(n); %切点横坐标
y0 = y(n);
k1 = (y(n)-y(n-1))/(x(n)-x(n-1));
k2 = (y(n+1)-y(n))/(x(n+1)-x(n));
k = (k1+k2)/2; %切线斜率
x1 = [-2*pi 2*pi]; %切线坐标
y1 = k*(x1-x0)+y0;
plot(x,y,'k-',x1,y1,'r-');
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