Question #148577
State whether the following statements are true or false. Justify your answer with a brief explanation or with a counter-example.

(a) The curve x4 + y4 = 4a2xy is symmetric with respect to the origin.

(b) The direction cosines of the line x = — y, z=0 are 1/√2, -1/√2,0

(c) The section of 2x² + y²= 2 (1 — z²) by the plane x + 2 = 0 is a hyperbola.-

(d) The xy-plane intersects the sphere

x²+ y² + z² + 2x + 2y — z = 2 in a great circle.

(e) If the projection of a line segment AB on another line is the line segment CD, then

IABI = ICDI•
1
Expert's answer
2020-12-04T10:58:34-0500

(a) If a point M(t,s)M(t,s) belong to the curve x4+y4=4a2xyx^4 + y^4 = 4a^2xy, then t4+s4=4a2tst^4 + s^4 = 4a^2ts. Then for the point M(t,s)M'(-t,-s) we have (t)4+(s)4=t4+s4=4a2ts=4a2(t)(s)(-t)^4 + (-s)^4 =t^4+s^4= 4a^2ts=4a^2(-t)(-s), and thus the point M(t,s)M'(-t,-s) is also belong to the curve x4+y4=4a2xyx^4 + y^4 = 4a^2xy. Consequently, the curve is symmetric with respect to the origin.


Answer: true


(b) Since the vector v=(1,1,0)v=(1,-1,0) is parallel to the line x=y, z=0x = - y,\ z=0 and v=2|v|=\sqrt{2}, we conclude that cosα=12, cosβ=12, cosγ=02=0.\cos \alpha =\frac{1}{\sqrt{2}}, \ \cos \beta =-\frac{1}{\sqrt{2}}, \ \cos \gamma =\frac{0}{\sqrt{2}}=0.


Answer: true


(c) The general equation of a hyperbola is


x2a2y2b2=1\frac{x^2}{a^2}-\frac{y^2}{b^2}=1.


Let us find the section of 2x2+y2=2(1z2)2x^2+y^2=2(1-z^2) by the plane x+2=0x+2=0:


2(2)2+y2=2(1z2)2(-2)^2+y^2=2(1-z^2)


8+y2=22z28+y^2=2-2z^2


y2+2z2=6y^2+2z^2=-6


Since the eqution has no real solution, the plane x+2=0x+2=0 does not intersect 2x2+y2=2(1z2)2x^2+y^2=2(1-z^2). The equation y2+2z2=6y^2+2z^2=-6 is not a hyperbola equation.


Answer: false


(d) The xy-plane intersects the sphere x2+y2+z2+2x+2yz=2x^2+y^2+z^2+2x+2y-z=2 in a great circle if and only if the center of this sphere belong to xyxy-plane. Let us rewrite the equation of the sphere in the the following form: (x+1)2+(y+1)2+(z12)2=2+1+1+14=174.(x+1)^2+(y+1)^2+(z-\frac{1}{2})^2=2+1+1+\frac{1}{4}=\frac{17}{4}. It follows that M(1,1,12)M(-1,-1,\frac{1}{2}) is the center of the sphere. Taking into account that the third coordinate of MM is not equal to 0, we conclude that the center of the sphere does not belong to the xyxy-plane, and therefore, the sphere x2+y2+z2+2x+2yz=2x^2+y^2+z^2+2x+2y-z=2 does not intersect the xyxy-plane in a great circle.


Answer: false


(e) Let consider any line segment ABAB with AB=2.|AB|=2. The projection of a line segment ABAB on another line is the line segment CDCD formed by the projections of the end points of the line segment ABAB on this line. If we choose such a line ll that the segment ABAB is perpendicular to ll, then the points A and B projects on the same point C=DC=D of the line ll, and therefore, CD=0|CD|=0.


Answer: false



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