JEE MAIN - Mathematics (2019 - 10th January Morning Slot - No. 16)
Let z1 and z2 be any two non-zero complex numbers such that $$3\left| {{z_1}} \right| = 4\left| {{z_2}} \right|.$$ If $$z = {{3{z_1}} \over {2{z_2}}} + {{2{z_2}} \over {3{z_1}}}$$ then :
$${\rm I}m\left( z \right) = 0$$
$$\left| z \right| = \sqrt {{17 \over 2}} $$
$$\left| z \right| =$$ $${1 \over 2}\sqrt {9 + 16{{\cos }^2}\theta } $$
Re(z) $$=$$ 0
Explanation
Given, $$3\left| {{z_1}} \right| = 4\left| {{z_2}} \right|$$
$$ \Rightarrow $$ $${{\left| {{z_1}} \right|} \over {\left| {{z_2}} \right|}} = {4 \over 3}$$
$$ \Rightarrow $$ $${{\left| {3{z_1}} \right|} \over {\left| {2{z_2}} \right|}} = {4 \over 3} \times {3 \over 2} = 2$$
As we know, for any compled number
$${{3{z_1}} \over {2{z_2}}} = {{\left| {3{z_1}} \right|} \over {\left| {2{z_2}} \right|}}$$(cos$$\theta $$ + i sin$$\theta $$)
= 2(cos$$\theta $$ + i sin$$\theta $$)
$$ \therefore $$ $${{2{z_2}} \over {3{z_1}}}$$ = $${1 \over {2\left( {\cos \theta + i\sin \theta } \right)}}$$
= $${1 \over {2\left( {\cos \theta + i\sin \theta } \right)}} \times {{\left( {\cos \theta - i\sin \theta } \right)} \over {\left( {\cos \theta - i\sin \theta } \right)}}$$
= $${\left( {{1 \over 2}\cos \theta - {i \over 2}\sin \theta } \right)}$$
Now, given
$$z = {{3{z_1}} \over {2{z_2}}} + {{2{z_2}} \over {3{z_1}}}$$
= 2(cos$$\theta $$ + i sin$$\theta $$) + $${\left( {{1 \over 2}\cos \theta - {i \over 2}\sin \theta } \right)}$$
= $${{5 \over 2}\cos \theta + {3 \over 2}i\sin \theta }$$
So, |z| = $$\sqrt {{{25} \over 4}{{\cos }^2}\theta + {9 \over 4}{{\sin }^2}\theta } $$
= $${1 \over 2}\sqrt {9 + 16{{\cos }^2}\theta } $$
z is neither purely real nor purely imaginary and |z| depends on $$\theta $$.
$$ \Rightarrow $$ $${{\left| {{z_1}} \right|} \over {\left| {{z_2}} \right|}} = {4 \over 3}$$
$$ \Rightarrow $$ $${{\left| {3{z_1}} \right|} \over {\left| {2{z_2}} \right|}} = {4 \over 3} \times {3 \over 2} = 2$$
As we know, for any compled number
$${{3{z_1}} \over {2{z_2}}} = {{\left| {3{z_1}} \right|} \over {\left| {2{z_2}} \right|}}$$(cos$$\theta $$ + i sin$$\theta $$)
= 2(cos$$\theta $$ + i sin$$\theta $$)
$$ \therefore $$ $${{2{z_2}} \over {3{z_1}}}$$ = $${1 \over {2\left( {\cos \theta + i\sin \theta } \right)}}$$
= $${1 \over {2\left( {\cos \theta + i\sin \theta } \right)}} \times {{\left( {\cos \theta - i\sin \theta } \right)} \over {\left( {\cos \theta - i\sin \theta } \right)}}$$
= $${\left( {{1 \over 2}\cos \theta - {i \over 2}\sin \theta } \right)}$$
Now, given
$$z = {{3{z_1}} \over {2{z_2}}} + {{2{z_2}} \over {3{z_1}}}$$
= 2(cos$$\theta $$ + i sin$$\theta $$) + $${\left( {{1 \over 2}\cos \theta - {i \over 2}\sin \theta } \right)}$$
= $${{5 \over 2}\cos \theta + {3 \over 2}i\sin \theta }$$
So, |z| = $$\sqrt {{{25} \over 4}{{\cos }^2}\theta + {9 \over 4}{{\sin }^2}\theta } $$
= $${1 \over 2}\sqrt {9 + 16{{\cos }^2}\theta } $$
z is neither purely real nor purely imaginary and |z| depends on $$\theta $$.
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