抛物线x^2=4Y 上有A(X1,Y1) B(X2,Y2)且X1*X2=-8 若有一点P(0,1)求1/PA+1/PB
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抛物线x^2=4Y 上有A(X1,Y1) B(X2,Y2)且X1*X2=-8 若有一点P(0,1)求1/PA+1/PB 的取值范围
x1^2=4y1,x2^2=4y2
PA=√[x1^2+(y1-1)^2]
=√[x1^2+(x1^2/4-1)^2]
=√(x1^4/16+x1^2/2+1)
=√(x1^2/4+1)^2
=x1^2/4+1
PB=√[x2^2+(y2-1)^2]
=x2^2/4+1
m=1/PA+1/PB
=1/(x1^2/4+1)+1/(x2^2/4+1)
=4/(x1^2+4)+4/(x2^2+4)
=4/(x1^2+4)+4/(x1^2/64+4)
=4/(x1^2+4)+4/(64/x1^2+4)
=1/(y1+1)+y1/(4+y1)
=(y1^2+2y1+4)/(y1^2+5y1+4)
m=(y1^2+2y1+4)/(y1^2+5y1+4)
(m-1)y1^2+(5m-2)y1+4m-4=0
m=1时,y1=0
0<m≠1时,因y1≥0
△=(5m-2)^2-16(m-1)^2=(9m-6)(m+2)≥0
(5m-2)/(m-1)≥0
(4m-4)/(m-1)≥0
所以
(m-2/3)(m+2)≥0,m≥2/3
(m-2/5)(m-1)≥0,m>1或0<m<2/5
所以
m>1
综上所述m≥1
即1/PA+1/PB≥1.
PA=√[x1^2+(y1-1)^2]
=√[x1^2+(x1^2/4-1)^2]
=√(x1^4/16+x1^2/2+1)
=√(x1^2/4+1)^2
=x1^2/4+1
PB=√[x2^2+(y2-1)^2]
=x2^2/4+1
m=1/PA+1/PB
=1/(x1^2/4+1)+1/(x2^2/4+1)
=4/(x1^2+4)+4/(x2^2+4)
=4/(x1^2+4)+4/(x1^2/64+4)
=4/(x1^2+4)+4/(64/x1^2+4)
=1/(y1+1)+y1/(4+y1)
=(y1^2+2y1+4)/(y1^2+5y1+4)
m=(y1^2+2y1+4)/(y1^2+5y1+4)
(m-1)y1^2+(5m-2)y1+4m-4=0
m=1时,y1=0
0<m≠1时,因y1≥0
△=(5m-2)^2-16(m-1)^2=(9m-6)(m+2)≥0
(5m-2)/(m-1)≥0
(4m-4)/(m-1)≥0
所以
(m-2/3)(m+2)≥0,m≥2/3
(m-2/5)(m-1)≥0,m>1或0<m<2/5
所以
m>1
综上所述m≥1
即1/PA+1/PB≥1.
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