The correct option is B I−d,II−b,III−c,IV−a
I. xcosθ+ysinθ=a.....(i)
xsinθ−ycosθ=b.....(ii)
Squaring and adding equations (i) and (ii), we get
[xcosθ+ysinθ]2+[xsinθ−ycosθ]2=a2+b2
x2cos2θ+y2sin2θ+2xysinθcosθ+x2sin2θ+y2cos2θ−2xysinθcosθ=a2+b2
x2[sin2θ+cos2θ]+y2[sin2θ+cos2θ]=a2+b2
x2+y2=a2+b2
which is (d).
II. x=secθ+tanθ
y=secθ−tanθ
xy=(secθ+tanθ)(secθ−tanθ)
xy=sec2θ−tan2θ
xy=1[∵1+tan2θ=sec2θ]
which is (b).
III. xsecθ+ytanθ=a ....... (iii)
xtanθ+ysecθ=b ......... (iv)
Squaring and then subtracting equation (iii) from (iv), we get
[x2sec2θ+y2tan2θ+2xysecθtanθ]−[x2tan2θ+y2sec2θ+2xysecθtanθ]=a2−b2
x2[sec2θ−tan2θ]−y2[sec2θ−tan2θ]=a2−b2
x2−y2=a2−b2
which is (c).
IV. x=cotθ+cosθ
y=cotθ−cosθ
Squaring both equations, we get
x2=cot2θ+cos2θ+2cotθcosθ ......... (v)
y2=cot2θ+cos2θ−2cotθcosθ ......... (vi)
Subtract equation (vi) from (v), we get
x2−y2=4cotθcosθ
Squaring both sides, we get
(x2−y2)2=16cot2θcos2θ ......... (vii)
Now, xy=(cotθ+cosθ)(cotθ−cosθ)
xy=cot2θ−cos2θ
xy=cos2θsin2θ[1−sin2θ]
xy=cot2θcos2θ ......... (viii)
Substitute (viii) in equation (vii), we get
(x2−y2)2=16xy
which is (a).
Therefore, I-d, II-b, III-c, IV-a