请参考
1
a+2b=(2/3+2*2/3,-1/3+2*2/3,4+2*(-3))
=(2,1,-2)|a+2b|=√5
a-b=(0,-1,7)|a-b|=5√2
(a+2b)-(a-b)=3b=(2,2,-9)|3b|=√85
cosθ=[|a+2b|^2+|a-b|^2-|3b|^2]/[-2*|a+2b|*|a-b|]
=(5+50-85)/-2*(√5*5√2)=3/√10
θ=arccos(3/√10)
2
x^2/a^2-y^2/b^2=1
tanPF1F2=1/2cosPF1F2=√[1/1+(tanPF1F2)^2]=2/√5
sinPF1F2=1/√5
PF1=2c*(2/√5)=(4/√5)cPF2=2c*(1/√5)=(2/√5)c
PF1-PF2=2aa=(1/√5)ce=c/a=√5
3
x^2/64-y^2/36=1
a^2=64a=8
PF1=17PF1-PF2=2a=16PF2=1
PF2-PF1=2a=16PF2=33
4
y=k(x+1)交x轴于M(-1,0)位于抛物线外Soab=(1/2)|x1-x2|*|y1-y2|
如果题目是y=k(x-1).M(1,0)Soab=(1/2)|OM|y1-y2|
y^2=xy=k(x-1)
k^2(x-1)^2=x
k^2x^2+(-2k^2-1)x+k^2=0
x1+x2=(2k^2+1)/k^2=1/k^2+2
x1x2=1
(x1-x2)^2=(1/k^2+2)^2-4=1/k^4+4/k^2
|x1-x2|=√(1/k^2+4)/k
|y1-y2|=k|x1-x2|=√(1/k^2+4)
S=√10=1*|y1-y2|*(1/2)|y1-y2|=2√10
1/k^2+4=40
k^2=1/36
k=1/6或k=-1/6
5
y=2x/3+3x^2/4-y|y|/9=1
y02交点
y>0x^2/4-y^2/9=1
y=2x/3+3(9-4/9)x^2-16x-72=0
判别式>0,1交点,(除去y