[tex]1. C ^{4} _{5} +A ^{4} _{5}
C ^{4} _{5}= \frac{5!}{4!*1!} =5
A ^{4} _{5} =\frac{5!}{1!} =120
5+120=125 [/tex] Am folosit formula Combinarilor si Aranjamentelor, iar apoi am simplificat.
[tex]1+ \frac{1}{3} + \frac{1}{3 ^{2} } + \frac{1}{ 3^{3} } + \frac{1}{ 3^4} } =1+ \frac{3 ^{4}+3^3+3^2+3+1 }{3^4} = \frac{81+27+9+3+1}{81} = \frac{121}{81} [/tex]. Am adus la acelasi numitor, acesta fiind 3^4, iar apoi am ridicat la putere.