x₁=-9; xn=xn-1+12; n=2;3;4...
d=12
xn=x₁+(n-1)*d=-9+(n-1)*12=-9+12n-12=12n-21
xn=12n-21
x₁=-9; x₂=3; x₃=15; x₄=27; x₅=39
x₁=-9; xn=xn-1+12; n=2;3;4...
d=12
xn=x₁+(n-1)*d=-9+(n-1)*12=-9+12n-12=12n-21
xn=12n-21
x₁=-9; x₂=3; x₃=15; x₄=27; x₅=39