a)
\(\displaystyle{ a _{25}= \frac{90-4 \cdot 25}{3} = \frac{90-100}{3}= -\frac{10}{3}}\)
b)
\(\displaystyle{ -6= \frac{90-4 \cdot n}{3}}\)
\(\displaystyle{ -18=90-4n}\)
\(\displaystyle{ -108=-4n}\)
\(\displaystyle{ n=27}\)-- 11 grudnia 2009, 13:22 --c)
\(\displaystyle{ \frac{90-4n}{3} >0}\)
\(\displaystyle{ 90-4n>0}\)
\(\displaystyle{ -4n>-90}\)
\(\displaystyle{ n<22 \frac{1}{2}}\)
czyli 22 wyrazy dodatnie