The ratio of the sides of two similar regular octagons is 5:7. If the measure of each side of the larger octagon is 21 cm, then the measure of each side of the smaller octagon is cm.

15 centimeters.

Step-by-step explanation:

If the ratio of two sides of similar octagons is 5:7, then the larger octagon has a side length that is 7/5 times the smaller one. If the measure of the side of the larger octagon is 21, then the measure of the smaller one is 5/7 times that, which is 5/7 * 21 = 15 centimeters.

the last choice

step-by-step explanation:

hello from mrbilldoesmath!

answer:

25x^2 + 90x + 84

discussion:

mn(x) = m(n(x)) = m( 5x + 9) =

(5x + 9)^2 + 3 =>

25x^2 + 90x + 9^2 + 3 =

25x^2 + 90x + 84                 (as 9^2 = 9*9 = 81)

which is the second choice

regards,  

mrb

p.s.   i'll be on vacation from friday, dec 22 to jan 2, 2019. have a great new year!