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**CALCULUS, EXTREME VALUES of function on interval and where they occur!!!**

find the extreme value of function on the interval and where they occur.

Please Help, teacher did during class but didnt get explanation.

1. y=cos(x-(pi/4)) , at the interval of [0,7pi/4]

My attempt: y'= -sin (x-(pi/4))

y= pi/4, 5pi/4

f(0)= -1/sqrt(2) f(7pi/4)= 0 f(pi/4)= 0 f(5pi/4)=-1

Max of 1 @ x=pi/4

Min of -1 at x=5pi/4

teacher said f(0) was wrong i dont know why

2. y= 4x/(x^2+1)

My attempt: y'= -4x/(x^2+1)^2

f(0)=0

Max of 0 at x=0

dont know why i got that wrong

tnx a lot

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