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Aratati ca a=(√3+√5)•(3-√15)+(1-√3)²+4√3 este natural

Răspuns :

[tex]a=(\sqrt3+\sqrt5)(3-\sqrt{15})+(1-\sqrt3)^2+4\sqrt3\\ a=3\sqrt3+3\sqrt5-\sqrt{45}-\sqrt75}+1-2\sqrt3+3+4\sqrt3\\ a=3\sqrt3+3\sqrt5-3\sqrt5-5\sqrt3+4+2\sqrt3\\ a=5\sqrt3-5\sqrt3+4\\ a=4 \in N[/tex]
le desfacem rand pe rand 
(rad3+ rad5)(3-rad15)=
3 rad3+3rad 5 -rad 45 -rad 75 =
3 rad3+3rad5-3 rad5 - 5 rad 3
3 rad 3 - 5 rad 3 +3 rad 5 -3 rad 5 =
-2 rad 3 

facem mai departe
(1- rad3)^2 = 
1-2 rad3 + 3=
1+3-2rad3=
4-2rad3

Le punem cap la cap

-2 rad3 +4 - 2 rad 3 +4 rad 3=
-4 rad3 + 4 rad 3 +4 = 
=4 NUMAR NATURAL