1.

The length of the projection of the line segment joining the points `(5,-1,4)` and `(4,-1,3)` on the plane `x+y+z=7` is

Answer» `A(5,-1,4) and B(4,-1,3)`
`AB=(4-5)veci + (-1+1)vecj + (3-4)veck`
`=-veci-veck`
`abs(AB)=sqrt2`
`x+y+z=7`
`n=(veci+vecj+veck)`
Equation of plane
`vecr*vecn=d`
`(xveci+yvecj+zveck)(veci+vecj+veck)=7`
`absn=sqrt3`
`P=ABsintheta`
`costheta =(vec(AB)*vecn)/(abs(AB)*absvecn)`
`=((-veci-veck)(veci+vecj+veck))/(sqrt2sqrt3)`
`=costheta=-sqrt2/sqrt3`
`sintheta=sqrt(1-cos^2theta)`
`sintheta=1/sqrt3`
`P=sqrt2*1/sqrt3=sqrt2/sqrt3`


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