Vectors and their product?

Two vectors lie with their tails at the same point. When the angle between them is increased by 20degr. the magnitude of their vector product doubles.


The original angle between them was about:


a 0


b 18degr


c 25 degr


d 45 degr


e 90 degr





I drew pictures for all answers and I think only 0 degr would work but I'm not sure. How can I calculate this?

Vectors and their product?
The vector product is proportional to the sine of the angle between the vectors. Therefore, you need to find an angle x such that sin(x + 20) = 2*sin(x). I'd suggest performing trial and error on the five choices to see which angle has a sine that doubles when you add 20 degrees. You should find that one of your choices gives you a result that is close to double, although it's not exact; none of the answers gives the exact result.





It's definitely not 0, because the original vectors would have a vector product of 0, and double that is still 0.
Reply:the answer is always c.
Reply:$1.98
Reply:MAGNITUDE OF THE VECTORS INCREASE 2 TIMES WHICH IS MAX AND THE ANGLE FAVOURNG THE MAX INCREASEIS 45 DEGREES .SO IT SHOULD HAVE BEEN INITIALLY AT AN ANGLE OF 25 AS LATER ON IT IS INCREASED BY 20 DEGREE


20+?=45


ANSWER=25