1.

Let `P`be a pointin the first octant, whose image `Q`in theplane `x+y=3`(that is,the line segment `P Q`isperpendicular to the plane `x+y=3`and themid-point of `P Q`lies in theplane `x+y=3)`lies on the z-axis. Let the distance of `P`from the x-axis be 5. If `R`is theimage of `P`in thexy-plane, then the length of `P R`is _______.

Answer» Correct Answer - 8
Let `P(alpha,beta,gamma)` and R is image of P in the xy-plane.
`:.R(alpha,beta,-gamma)`
Also, Q is the imgae of P in the plane `x+y=3`
`:.(x-alpha)/(1)=(y-beta)/(1)=(z-gamma)/(0)=(-2(alpha+beta-3))/(2)`
`x=3-beta,y=3-alpha,z=gamma`
Since, Q is lies on Z-axis
`:." "beta=3,alpha=3,z=gamma`
`:.P(3,3,gamma)`
Given, distance of P from X-axis be 5.
`:." "5=sqrt(3^(2)-gamma^(2))`
`25-9=gamma^(2)`
`impliesgamma=+-4`
Then, `" "PR=|2gamma|=|2xx4|=8`


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