f fonction homographique ==> f(x)=(ax+b)/(cx+d)
1) m= (a-d)/2c si ( c#0) . f(m)=m <==> am+b=cm²+dm
2) 1/(f(x)-m) = (cx+d)/ (ax+b- m(cx+d))
ax+b- m(cx+d)=(a- mc)x+b-md =(a- mc)x+cm²-am car am+b=cm²+dm
==> ax+b- m(cx+d)= (a- mc)(x-m)
cx+d= c(x-m)+cm+d = c(x-m)+ a-mc car m= (a-d)/2c <==> a-mc=cm+d
==> 1/(f(x)-m) = c/(a-mc) +1/(x-m) ==> k= c/(a-mc)=c/(cm+d)
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