Wyznacz wzór ogólny ciągu arytmetycznego jeżeli a5=4 i a21=8

a5=4     a21=8 a5+16r=a21 4+16r=8 16r=4 |:16 r=4/16 r=1/4 a1=a5-4r a1=4-( 4 x 1/4 ) a1=4-1 a1=3 An=3+(n-1) x 1/4 An=3+1/4n-1/4 An=2 i 3/4 + 1/4n liczę na naj;))

[latex]a_{n}=a_{1}+(n-1)r[/latex] [latex]a_{21}=8=a_{1}+(21-1)r[/latex] [latex]a_{5}=4=a_{1}+(5-1)r[/latex] [latex]a_{21}=8=a_{1}+(21-1)r[/latex] [latex]a_{5}=4=a_{1}+(5-1)r[/latex] [latex]8=a_{1}+20r[/latex] [latex]a_{1}=4-4r[/latex] [latex]8=4-4r+20r[/latex] [latex]r =frac{1}{4}a_{1}=3[/latex] [latex]a_{n}=3+frac{1}{4}(n-1)[/latex]