Guest · filters and generic practice availableBrowsing as a guest · PYQ filters and generic practice are available. Sign in only for personalised features and saved progress.
Let w=zzˉ+k1z+k2iz+λ(1+i),k1,k2∈R. Let Re(w)=0 be the circle C of radius 1 in the first quadrant touching the line y=1 and the y-axis. If the curve Im(w)=0 intersects C at A and B, then 30(AB)2 is equal to
Numerical answer
View written solutionFree
Correct answer: 120
Let z=x+iy with x,y∈R. Then
zˉ=x−iy,zzˉ=x2+y2.
We are given
w=zzˉ+k1z+k2iz+λ(1+i),k1,k2∈R.
We must use the condition that Re(w)=0 is a circle of radius 1 in the first quadrant touching the line y=1 and the y-axis.
1. Write Re(w) and Im(w)
First compute:
k1z=k1x+ik1y,iz=i(x+iy)=ix−y,
so
k2iz=−k2y+ik2x.
Also,
λ(1+i)=λ+iλ.
Hence
w=(x2+y2)+(k1x−k2y+λ)+i(k1y+k2x+λ).
The curve Re(w)=0 is
x2+y2+k1x−k2y+λ=0.
This is a circle.
For a circle
x2+y2+2gx+2fy+c=0,
center is (−g,−f) and radius is g2+f2−c.
Comparing,
2g=k1⇒g=2k1,2f=−k2⇒f=−2k2.
So center is
(−2k1,2k2).
Now the circle has radius 1, lies in the first quadrant, touches the line y=1 and the y-axis (x=0).
A circle of radius 1 touching x=0 must have center at distance 1 from the y-axis, so
xext−coordinateofcenter=1.
A circle of radius 1 touching y=1 and lying below it in the first quadrant must have center at distance 1 from y=1, hence
y-coordinate of center=0.
Thus center is (1,0) and radius is 1.
So the circle is
(x−1)2+y2=1,
or
x2+y2−2x=0.
Comparing with
x2+y2+k1x−k2y+λ=0,
we get
k1=−2,k2=0,λ=0.
3. Equation of Im(w)=0
Now
Im(w)=k1y+k2x+λ=0.
Substitute the values:
−2y=0⇒y=0.
So the curve Im(w)=0 is the x-axis.
4. Intersection of this line with the circle
Circle:
(x−1)2+y2=1.
With y=0,
(x−1)2=1
so
x−1=±1⇒x=0,2.
Hence the intersection points are
A=(0,0),B=(2,0).
So
AB=2.
Therefore,
30(AB)2=30⋅4=120.
5. Compare with stored answer
My derived answer is
120.
The stored correct answer is 24.
These do not match. The geometry and algebra both consistently give AB=2, hence 30(AB)2=120.