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Progresie aritmetica
Posted: Tue Jan 13, 2009 6:45 pm
by Al3x
\( a_2+a_{40}=16 \)
\( a_5=28 \)
\( a_1=?,\ r=? \)
Posted: Tue Mar 30, 2010 8:14 pm
by Alin
\( \ a_2+\ a_{40}=16 \) adica \( \ 2a_1+40r=16 => \ a_1+20r=8 (1) \). Din \( \ a_5=28 \) scoatem \( \ a_1+4r=28 (2) \). Din \( (1), (2) => 16r=-20 => r=-\frac {20}{16}=-\frac {5}{4} \). Stim ratia si \( \ a_5=28 \) si aflam ca \( \ a_1=33 \).