F(x) = sin(-3x) increase decrease intervals

  • Thread starter vaze
  • Start date
  • #1
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Hi everyone.

Could anyone help me solve this. I need to find the increase and decrease intervals of f(x) = sin(-3x).

The increase intervals of sin(x) are [itex]-\pi/2 + 2\pi*k < x < \pi/2 + 2\pi*k[/itex]

Is the following the right way to solve my problem (for increase intervals)? [itex]-\pi/2 + 2\pi*k < -3x < \pi/2 + 2\pi*k[/itex]
By multiplying by -3 you get: [itex]-\pi/6 + 2/3\pi*k < x < \pi/6 + 2/3\pi*k[/itex] (1)

Although when plotting sin(-3x) this interval (1) is the decrease one. Why?
 

Answers and Replies

  • #2
mathman
Science Advisor
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sin(-3x)=-sin(3x). Work with this form, since you don't have to work with things going in opposite directions.
 
  • #3
3
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How would you use sin(x) = -sin(x) in this problem?
 
  • #4
692
142
How would you use sin(x) = -sin(x) in this problem?
If sin(x) is increasing then -sin(x) is decreasing, vice versa.
 
  • #5
3
0
Yes, thank you. I have already solved it this way. My textbook had an error in the solutions sheet. They just solved the double inequality and did not exchange the increase and decrease intervals.
 

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