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If `theta` is the angle between the unit vectors `veca and vecb`, then prove that i. `cos((theta)/2)=1/2|veca+vecb|`` ii. sin (theta/2)=1/2|veca-vecb|` |
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Answer» i. `(veca+vecb).(veca+vecb)= |veca|^(2)+|vecb|^(2)+2veca.vecb` `=1 +1+2(1)(1) cos theta` `cos(theta/2)=1/2|veca+vecb|` ii. `(veca-vecb).(veca-vecb)=|veca|^(2)=|vecb|^(2)-2veca.vecb` `= 1+1-2(1)(1)cos theta` `=2-2costheta` `or sin"theta/2=1/2|veca+vecb|` |
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