• Dec 21st 2012, 11:06 AM
Vince604
Solve Each System:

x2 + y2 = 20
y = x2

This might be a dumb question question, but 20 isn't a square number, so how do i find the radius to graph a circle? Please help!
• Dec 21st 2012, 11:11 AM
Plato
Quote:

Originally Posted by Vince604
Solve Each System:

x2 + y2 = 20
y = x2

This might be a dumb question question, but 20 isn't a square number, so how do i find the radius to graph a circle? Please help!

It has $r=$ $\sqrt{20}$
• Dec 21st 2012, 11:31 AM
HallsofIvy
No, it doesn't. It has radius sqrt{20}= sqrt{4(5)}= 2 sqrt{5} which is about 4.472.

(Been hitting the eggnog a little early?)
• Dec 21st 2012, 11:57 AM
Soroban
Hello, Vince604!

Quote:

Solve:
. . x2 + y2 = 20 . [1]
. . . . y = x2 . . . [2]

Substitute [2] into [1]: .y + y2 = 20 . . y2 + y - 20 = 0

. . . (y - 4)(y + 5) .= .0 . . y = 4, -5

Substitute into [2]:

. . 4 = x2 . . x = +2

. .-5 = x2 . No real roots

Therefore: .(x, y) .= .(+2, 4)
• Dec 22nd 2012, 09:24 AM
Vince604
Quote:

Originally Posted by Soroban
Hello, Vince604!

Substitute [2] into [1]: .y + y2 = 20 . . y2 + y - 20 = 0

. . . (y - 4)(y + 5) .= .0 . . y = 4, -5

Substitute into [2]:

. . 4 = x2 . . x = +2

. .-5 = x2 . No real roots

Therefore: .(x, y) .= .(+2, 4)

Thanks, it makes a lot of sense now.