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    				| 1. | Prove that a necessaryand sufficient condition for three vectors ` vec a , vec b`and ` vec c`to becoplanar is that there exist scalars `l , m , n`not all zerosimultaneously such that `l vec a+m vec b+n vec c= vec0dot` | 
| Answer» Here, it is given that , `lveca+mvecb+nvecc = 0` If we put, `l =-1 , m = x and n = y`, then, `-veca +xvecb+yvecc = 0` `=> veca = xvecb+yvecc`, which is the neccessary condition for all three vectors to be coplanar. Now, `lveca+mvecb+nvecc = 0` `=>lveca = -mvecb-nvecc` `=>veca = -m/lvecb-n/lvecc`, which is the sufficient condition for all three vectors to be coplanar. | |