Find local max

Calculus Level 2

find local max then find range abs (sinx)+abs(cosx)


The answer is 0.4142.

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2 solutions

Local maximum of sin x + cos x \sin x+|\cos x| is obtained as sin x + cos x = 2 sin ( x + π 4 ) \sin x+\cos x=\sqrt 2 \sin (x+\frac{π}{4}) in the range, for example, [ 0 , π ] [ 2 π , 3 π ] . . . [ 2 n π , ( 2 n + 1 ) π ] . . . [0, π]\cup [2π,3π]\cup... \cup [2nπ,(2n+1)π]\cup... . The maximum value is thus 2 1.4142 \sqrt 2 \approx \boxed {1.4142} .

Oh, I missed the abs before sin x \sin x .

In this case, 0 sin x + cos x 2 0\leq |\sin x|+|\cos x|\leq \sqrt 2 \implies maximum value of the quantity is 2 \sqrt 2 and the range is [ 0 , 2 ] [0,\sqrt 2 ] .

Chew-Seong Cheong
May 10, 2020

The blue curve is sin x + cos x \sin x + \cos x and the red curve is sin x + cos x |\sin x|+|\cos x| . Should the range of sin x + cos x |\sin x|+|\cos x| be 2 1 0.414 \sqrt 2-1 \approx 0.414 ?

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