Polar Multiplication
If two complex numbers have moduli 2 and 5 and arguments 40 degrees and 80 degrees respectively, then the modulus and argument of their product are correctly given by which pair?
Select the correct option:
Solution
modulus 10, argument 120 degrees
Multiplying two complex numbers in polar form multiplies their moduli and adds their arguments, a defining property exploited throughout JEE Advanced. With moduli 2 and 5, the product modulus is 2·5 = 10. With arguments 40 degrees and 80 degrees, the product argument is 40 + 80 = 120 degrees, which already lies in the principal range. Hence the product has modulus 10 and argument 120 degrees. Option modulus 7 wrongly adds the moduli instead of multiplying. Option argument 40 degrees forgets to add the second angle. Option modulus 3 and argument 3200 degrees confuses subtraction and multiplication of the angles. Hence the product is modulus 10 at 120 degrees. Plausibility check: the modulus of a product always equals the product of moduli, so 10 is forced, and adding arguments rotates the result, giving 120 degrees, both consistent with the geometric scaling-and-rotation interpretation of complex multiplication.
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About This Question
- Subject
- mathematics
- Chapter
- complex numbers and quadratic equations
- Topic
- polar multiplication
- Difficulty
- Easy
- Year
- 2025
Solution
Correct Answer:
modulus 10, argument 120 degrees
Multiplying two complex numbers in polar form multiplies their moduli and adds their arguments, a defining property exploited throughout JEE Advanced. With moduli 2 and 5, the product modulus is 2·5 = 10. With arguments 40 degrees and 80 degrees, the product argument is 40 + 80 = 120 degrees, which already lies in the principal range. Hence the product has modulus 10 and argument 120 degrees. Option modulus 7 wrongly adds the moduli instead of multiplying. Option argument 40 degrees forgets to add the second angle. Option modulus 3 and argument 3200 degrees confuses subtraction and multiplication of the angles. Hence the product is modulus 10 at 120 degrees. Plausibility check: the modulus of a product always equals the product of moduli, so 10 is forced, and adding arguments rotates the result, giving 120 degrees, both consistent with the geometric scaling-and-rotation interpretation of complex multiplication.
This easy difficulty mathematics question is from the chapter complex numbers and quadratic equations, covering the topic of polar multiplication. It appeared in the 2025 exam.
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