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C3 question on minimum values

Okay so I'm having so much trouble trying to. Find the minimum value when we have an equation in the form Rcos(theta -alpha)
One paper I do and the mark scheme says to equal this function to zero. Then I do another paper but this time it equals 1 to find minimum. Is there a method on trying to get these questions correct?


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Avatar for KaylaB
KaylaB
20
Original post by Lilly1234567890
Okay so I'm having so much trouble trying to. Find the minimum value when we have an equation in the form Rcos(theta -alpha)
One paper I do and the mark scheme says to equal this function to zero. Then I do another paper but this time it equals 1 to find minimum. Is there a method on trying to get these questions correct?


Posted from TSR Mobile


In general to find a maximum or minimum point of a function you have to differentiate it, then make it equal zero
Reply 2
Avatar for KaylaB
KaylaB
20
Original post by Lilly1234567890
Okay so I'm having so much trouble trying to. Find the minimum value when we have an equation in the form Rcos(theta -alpha)
One paper I do and the mark scheme says to equal this function to zero. Then I do another paper but this time it equals 1 to find minimum. Is there a method on trying to get these questions correct?


Posted from TSR Mobile


I have just had a second to actually think about and use my brain to read the question, for these questions it's best to think about it graphically.

On the normal cos graph, we know that maximum point is 1 and minimum point is at -1.

We know then that if we do the transformation 2cos(x) (the same as doing 2f(x) where f(x) = cos(x)) that the maximum point would be 2 and minimum would be -2. So for Rcos(x) maximum would be R and minimum would be -R.

Could you then use this to figure out the first place where Rcos(x-alpha) = -R ?

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