圆（Circle）是一个常见和熟悉的曲线。
圆心在原点、半径为a的圆的一般方程为:
圆的图形：
with(plots):a:=1:
curve:=plot([a*cos(theta),a*sin(theta),theta=0..2*Pi],thickness=2):
display(curve,scaling=constrained,tickmarks=[0,0]);
动画：
with(plots):a:=1:
x:=t->a*cos(t):y:=t->a*sin(t):
K:=100:for i from 1 to K do ti:=i*2*Pi/K:
yuan[i]:=plot([x(t),y(t),t=0..ti],thickness=3):
xian[i]:=plot([[0,0],[x(ti),y(ti)]],color=blue,thickness=3) od:
yuan:=display(seq(yuan[i],i=1..K),insequence=true):
xian:=display(seq(xian[i],i=1..K),insequence=true):
display(yuan,xian,scaling=constrained,tickmarks=[4,4],view=[-1.2..1.2,-1.2..1.2]);
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