What is the radius of a circle whose equation is x2 + y2 + 8x – 6y + 21 = 0? units

To answer this, use "completing the square."

x2 + y2 + 8x – 6y + 21 = 0 becomes

x^2 + 8x + 16 - 16 + y^2 - 6y + 9 - 9 + 21 = 0

 (x + 4)^2       -16     + (y - 3)^2     + 12 = 0

or   (x + 4)^2      + (y - 3)^2   = 4

Since 4 = r^2, r = 2 units, and the center is at (-4, 3)

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!

explanation:

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