a, b cifre
[tex] \frac{abab}{2} = \frac{baba}{9}
[/tex]
abab=1000a+100b+10a+b=1010a+101b
baba= 1010b+101a
inlocuind in proportie si aplicand teorema fundamentala a proportiei (prod mezi=prod extremi) obtinem
9(1010a+101b)=2(1010b+1010a)
9090a+909b=2020b+202a
9090a-2020a=2020b-909b
8888a=1111b
8a=b
a si b cifre ⇒a=1 b=8
a+b=1+8=9
