If a circle has an area of 84π units then what is the measure of its radius in exact units?
a. 2√21 units
b. √ 54 units
C 10.4 units
d. 108 units

Answer quickly please

a. 2√21 units

Hi there !

Area formula

A = π·r²

84π = πr²

r² = 84π/π

r² = 84

r² = 2²×3×7

r = √2²×3×7

r = 2√3×7

r = 2√21

Good luck !

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!