(f°g)(x)=f(g(x))= 9 / ((4/x) - 5) = 9 / ( (4 - 5x)/ x) = 9x / (4 - 5x ) because a/(b/c) = ac/b

D={xc R / '4 - 5x =/= 0 } so : x =/= 4/5

so : D= ]-infini , 4/5 [ U ] 4/5 , +infini [

you have a good answer you are perfect in maths

so good luck

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