Answer:
Option (C) is correct.
Step-by-step explanation:
Given that z = 0.3(cos(31°) + i sin(31°)) and 20(cos(18°) + i sin(18°)).
The product of both the complex number is
zw = [0.3(cos(31°) + i sin(31°))] x [20(cos(18°) + i sin(18°))]
=(0.3x20)[cos(31°) x cos(18°) + cos(31°) x i sin(18°) + i sin(31°) x cos(31°)+ i sin(31°) x i sin(18°)]
Here, (0.3x20)=6, is the magnitude of zw, which is stretching of 0.3 by 20.
zw=6[cos(31°) x cos(18°) + i{cos(31°) x sin(18°) + sin(31°) x cos(31°)}+ i² sin(31°)x sin(18°)]
As i²= -1, So
zw=6[cos(31°) x cos(18°) + i{cos(31°) x sin(18°) + sin(31°) x cos(31°)} - sin(31°)x sin(18°)]
=6[{cos(31°) x cos(18°)- sin(31°)x sin(18°)} + i{cos(31°) x sin(18°) + sin(31°) x cos(31°)}]
As cosAcosB - sin A sin B = cos (A+B) and cosAsinB + sin A sos B = sin (A+B), so
zw=6[cos(31°+18°) + i{sin(31°+18°)]
Note that, initially the argument of z is 31° and after rotation of 18° in counterclockwise direction, the argument od zw is 31°+18°= 49°. i.e
zw=6(cos(49°) + isin(49°)).
So, zw can be determined by stretching z by a factor of 20, then rotating by 18° counterclockwise.
Hence, option (C) is correct.
