Which is part of the graph of y = − x cos x ?
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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 2 − π < x < 0 , − x > 0 , cos x > 0
Hence the graph where the y values are < 0 , for x ∈ ( − 2 π , 0 ) , are excluded.
When 0 < x < 2 π , − x < 0 , cos x > 0
Hence the graph where the y values are > 0 , for x ∈ ( 0 , 2 π ) , are excluded.
Option D is all that is left.
So let us analyse the graph.
At x=0, y=0 and all the graphs satisfy this. At x= Π / 2 , y=- Π / 2 cos( Π / 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 .
At x= Π / 4 , y=- Π / 4 cos( Π / 4 ) = - Π / ( 4 2 ), which is a negative value. so, A and B are eliminated.
Now at x=- Π / 4 , y = Π / 4 cos(- Π / 4 ) = Π / 4 cos( Π / 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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y = − x cos x is an odd function, so its graph is symmetrical about ( 0 , 0 ) , therefore A and C are impossible.
Furthermore, when x ∈ ( 0 , 2 π ) , y = − x cos x < 0 .