Algebra and Trigonometry 10th Edition

$a_n=100-9n$
$a_1=91=100-9=100-9(1)$ $a_2=82=100-18=100-9(2)$ $a_3=73=100-27=100-9(3)$ $a_4=64=100-36=100-9(4)$ $a_5=55=100-45=100-9(5)$ Each term is $9n$ less than 100: $a_n=100-9n$