Fig169.tex

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\multido{\i=-10+1}{21}{%
    \parametricplot{-6}{6}{%
        /c 0.1 def
        /b 4.1 def
        /X t def
        /Y X 80 dup sin exch cos div mul
           b \i\space mul c \i\space dup mul mul sub
        sub
        def
        X Y}}
\multido{\i=-10+1}{21}{%
    \parametricplot{-6}{6}{%
        /c 0.1 def
        /b 4.1 def
        /X t def
        /Y X 80 dup sin exch cos div mul neg
           b \i\space mul c \i\space dup mul mul add
        add
        def
        X Y}}
% paraboles
\multido{\i=-4+1}{8}{%
 \parametricplot[linecolor=red]{-6}{6}{%
        /c 0.1 10 sin mul def
        /b 4.1 10 sin mul def
        /X t def
        /Y c X dup mul mul 10 cos dup mul mul
        b c \i\space mul add dup mul 10 sin mul div
        b \i\space mul 2 div 10 sin div
        add
        \i\space dup mul c mul 4 div 10 sin div
        add
        def
        X Y}}
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\end{psclip}
\rput(-7.5,0){%
    \psline(0,-2)(0,4)
    \psline(-1,0)(1,0)
    \uput[90](0,4){$y$}
    \uput[90](1,0){$x$}
    \psline(4;80)
    \psline(4;100)
    \uput[0](4;80){$\mathrm{S_1}$}
    \uput[180](4;100){$\mathrm{S_2}$}
    \uput[225](0,0){O}
    \psarc(0,0){2}{80}{90}
    \psarc(0,0){1.8}{90}{100}
    \uput[85](2;85){$\THETA$}
    \uput[95](1.8;95){$\THETA$}
    }
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\end{center}