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Can't figure out this question

Its from the c3 solomon paper A and the question 4 part b. Whenever i put in x=pi/4 i keep getting a different answer for y to what it says on the mark scheme. Can someone explain how to do it?

Thanks
Original post by ~amethyst~
Its from the c3 solomon paper A and the question 4 part b. Whenever i put in x=pi/4 i keep getting a different answer for y to what it says on the mark scheme. Can someone explain how to do it?

Thanks


Just to make sure, have you differentiated y before you put x=pi/4 into dy/dx ?
Reply 2
Original post by randlemcmurphy
Just to make sure, have you differentiated y before you put x=pi/4 into dy/dx ?


yep, i got dy/dx= 2xsec^2x+2tanx

I don't see where im going wrong afterwards...i might have made a silly mistake somewhere, but i tried putting x in both rad and deg mode and still couldnt get the right answer.
Original post by ~amethyst~
yep, i got dy/dx= 2xsec^2x+2tanx

I don't see where im going wrong afterwards...i might have made a silly mistake somewhere, but i tried putting x in both rad and deg mode and still couldnt get the right answer.


You need to work out the equation of the tangent at the point, so put x=pi/4 into the original function (y), to get the corresponding y value for when x=pi/4, then work out dy/dx at pi/4 (make sure you are in radians)

Then use y-y1=m(x-x1)

Form the equation and it meets the y axis when x=0. So then work out y.
Reply 4
Original post by randlemcmurphy
You need to work out the equation of the tangent at the point, so put x=pi/4 into the original function (y), to get the corresponding y value for when x=pi/4, then work out dy/dx at pi/4 (make sure you are in radians)

Then use y-y1=m(x-x1)

Form the equation and it meets the y axis when x=0. So then work out y.


Oh okay, so is the grad=5.141592... correct? That's what i kept getting.
Original post by ~amethyst~
Oh okay, so is the grad=5.141592... correct? That's what i kept getting.


I've not actually done the question, but give it a go and see if it works, if not consult the markscheme again.
Reply 6
Original post by randlemcmurphy
I've not actually done the question, but give it a go and see if it works, if not consult the markscheme again.


I checked and yes finally got the answer. Thanks for your help :smile:
Gave it a go myself, did you get y = -1/4(pi)^2

So k = -1/4

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