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Geometry Level 3

Which is part of the graph of y = x cos x y=-x \cos x ?

B A D C None of these

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4 solutions

Jessica Wang
Jul 18, 2015

y = x cos x y=-x\cos x is an odd function, so its graph is symmetrical about ( 0 , 0 ) (0,0) , therefore A and C are impossible.

Furthermore, when x ( 0 , π 2 ) x\in (0,\frac{\pi}{2}) , y = x cos x < 0 y=-x\cos x< 0 .

Geoff Pilling
Nov 8, 2018

When you zoom in close to zero, y= -x. (d) is the only one with this behavior.

Since there are options given., my solution is going to be very short.

When π 2 < x < 0 , x > 0 , cos x > 0 \dfrac{-\pi}{2} <x < 0 , -x > 0 , \cos x > 0

Hence the graph where the y y values are < 0 < 0 , for x ( π 2 , 0 ) x \in (-\dfrac{\pi}{2} , 0) , are excluded.

When 0 < x < π 2 , x < 0 , cos x > 0 0 < x < \dfrac{\pi}{2}, -x < 0 , \cos x > 0

Hence the graph where the y y values are > 0 > 0 , for x ( 0 , π 2 ) x \in (0,\dfrac{\pi}{2}) , are excluded.

Option D is all that is left.

Nelson Mandela
Jul 18, 2015

So let us analyse the graph.

At x=0, y=0 and all the graphs satisfy this. At x= Π / 2 \Pi /2 , y=- Π / 2 \Pi/2 cos( Π / 2 \Pi/2 ) = 0 and the graphs seems so satisfy this even though the scale is not mentioned. Now for the critical analysis let us take Π / 4 \Pi /4 .

At x= Π / 4 \Pi /4 , y=- Π / 4 \Pi /4 cos( Π / 4 \Pi /4 ) = - Π / ( 4 2 \Pi /(4\sqrt { 2 } ), which is a negative value. so, A and B are eliminated.

Now at x=- Π / 4 \Pi /4 , y = Π / 4 \Pi /4 cos(- Π / 4 \Pi /4 ) = Π / 4 \Pi /4 cos( Π / 4 \Pi /4 ) as cos(-x) = cos(x).

so y is positive in this case and thus C is eliminated and answer is D. And as it cos is a periodic function, -xcosx will also be periodic.

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