1.

A square ABCD of diagonal =2a is folded along diagonal AC, so that planes DAC & BAC are at right angle. The shortest distance between DC & AB in the rew position is

Answer»

`sqrt(2)a`
`2a//sqrt(3)`
`2a//sqrt(5)`
None

Solution :Let
`F(X)=x("ln" x-1)`
`f(x) = "ln"x`
`f''(x)=(1)/(x)"",""underset(1)OVERSET(e)(int)((f''+f)sinx)dx`
Use integration by parts =`INTF''.sinx+intf.sinx`
`=sinx.f']_(1)^(e)-intcosx.f'+f.(-cosx)]_(1)^(e)-intf'.(-cosx)=sinx. "ln"x]_(1)^(e)-x("ln"x-1).(cosx)]_(1)^(e)`


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