B4
ๅณๅฝขใจๆน็จๅผ (40็น)ใฎ
ๅบงๆจๅนณ้ขไธใซๅ C:x+y-8x-6y+20=0 ใจ, ๅCใฎไธญๅฟใ้ใ็ด็ทl:y=--
-1/2x+k
x2+y^-8x-6y+20โฆ0
ใใใใ้ฃ็ซไธ็ญๅผ
ใฎ่กจใ้ ๅใDใจใใใ ใพใ, ไธญๅฟใ
2-1/2x+k
็น (a, 0), ๅๅพใใฎๅใKใจใใใ ใใ ใ, k, a,rใฏๅฎๆฐใจใ,r> 0 ใจใใใ
(1)ใฎๅคใๆฑใใใใพใ,ๅCใจ็ด็ทlใฎๅ
ฑๆ็นใฎๅบงๆจใๆฑใใใ
(2)a=0 ใจใใใ ๅKใจ้ ๅDใๅ
ฑๆ็นใใใคใจใ,rใฎๆๅฐๅคใจใใฎใจใใฎๅ
ฑๆ็นใฎ
ๅบงๆจใๆฑใใใ ใพใ, ๅKใจ้ ๅDใๅ
ฑๆ็นใใใคใจใ, rใฎๆๅคงๅคใๆฑใใใ
(3) 0โฆaโฆ10ใจใใใ ๅKใจ้ ๅDใๅ
ฑๆ็นใใใคใจใ, rใฎๆๅฐๅคใฮฑใฎๅคใซใใฃใฆ
ๅ ดๅๅใใใใฆๆฑใใใ