根号(1-(8/17)^2)
=根号(1-8^2/17^2)
=根号(1-8^2/17^2)
=根号(（17^2-8^2）/17^2)
=根号(225/17^2)
=根号(15^2/17^2)
=根号(15/17)^2
=15/17