For #z_1 = -3 + 2i# and #z_2=4+3i#, write #z_1/z_2# in polar form. In which quadrant will it lie in an Argand Diagram?

1 Answer
May 1, 2017

#z_1/z_2=sqrt13/5(cos(alpha-beta)+isin(alpha-beta))#, where #alpha-beta=tan^(-1)(1/6)# and lies in #Q1#

Explanation:

As #z_1=-3+2i#, we have #|z_1|=sqrt(3^2+2^2)=sqrt13#

and hence #z_1=-3+2i=sqrt13(-3/sqrt13+i2/sqrt13)#

= #sqrt13(cosalpha+isinalpha)#, where #cosalpha=-3/sqrt13# and #sinalpha=2/sqrt13#

Observe that #alpha# lies in #Q2# and #tanalpha=-2/3#

Similarly #z_2=4+3i#, we have #|z_1|=sqrt(4^2+3^2)=sqrt25=5#

and hence #z_2=4+3i=5(4/5+i3/5)#

= #5(cosbeta+isinbeta)#, where #cosbeta=4/5# and #sinbeta=3/5#

Observe that #beta# lies in #Q1# and #tanbeta=3/4#

Hence #z_1/z_2=(sqrt13(cosalpha+isinalpha))/(5(cosbeta+isinbeta))#

= #(sqrt13(cosalpha+isinalpha))/(5(cosbeta+isinbeta))xx(cosbeta-isinbeta)/(cosbeta-isinbeta)#

= #sqrt13/5xx((cosalphacosbeta+sinalphasinbeta)+i(sinalphacosbeta-cosalphasinbeta))/(cos^2beta+sin^2beta)#

= #sqrt13/5(cos(alpha-beta)+isin(alpha-beta))#

Now #tan(alpha-beta)=(tanalpha-tanbeta)/(1+tanalphatanbeta)#

= #(-2/3+3/4)/(1+(-2/3)xx(3/4))#

= #(1/12)/(1-2/4)=1/12xx4/2=1/6# and #alpha-beta=tan^(-1)(1/6)#

As #alpha# lies in #Q2#, #beta# lies in #Q1# and #tan(alpha-beta)>0#

Argand of #z_1/z_2# is in #Q1#