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May 6, 2016
Posted by **Cathy** on Sunday, June 23, 2013 at 6:19am.

- Physics -
**MathMate**, Sunday, June 23, 2013 at 6:35amA=[5,-6.5]

B=[-3.5,7]

If A is perpendicular to C, then A.C=0, or

C=[6.5k, 5k]

(verify that A.C=0)

If in addition, C.B=15, we have

C.B=[6.5k,5k].[-3.5,7]

=35k-22.75k

=12.25k

But C.B=15, therefore 12.25k=15, =>

k=15/12.25=60/49

Therefore

C=[6.5k,5k]

=[390/49, 300/49]

Check my arithmetic and that the given conditions are satisfied. - Physics -
**Cathy**, Sunday, June 23, 2013 at 7:41amthanks

- Physics :) -
**MathMate**, Sunday, June 23, 2013 at 8:14amyou're welcome!

- Physics -
**Charles**, Saturday, August 30, 2014 at 6:14pmI don't understand the flow in the math here. Please help me understand this solution by "dumbing it down" a bit.