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# Please help with vectors (I feel like it should be easy, but still can't get it) watch

1. I showed c.c' = 0 and I can't figure out the very last part. Any help is much appreciated.

EDIT: After hours of trying different combinations of substitutions I got it. My solution was really lengthy though. I first showed a^2 + a'^2 = b^2 + b'^2 which was alright and then that a^2 + a'^2 = c^2 + c'^2 which took a whole page of long lines. Anyone know a better/easier/quicker way?
I started by saying a.a' = b.b' and doing substitutions until I got the result and then the same thing with
a.a' = c.c' which as I said took ages.
Even if this is the right way, I had to do so many random combinations of substitutions until I got it and this doesn't seem very mathematical. Any other ideas? Thanks
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2. (Original post by monk1324)
I showed c.c' = 0 and I can't figure out the very last part. Any help is much appreciated.

EDIT: After hours of trying different combinations of substitutions I got it. My solution was really lengthy though. I first showed a^2 + a'^2 = b^2 + b'^2 which was alright and then that a^2 + a'^2 = c^2 + c'^2 which took a whole page of long lines. Anyone know a better/easier/quicker way?
I started by saying a.a' = b.b' and doing substitutions until I got the result and then the same thing with
a.a' = c.c' which as I said took ages.
Even if this is the right way, I had to do so many random combinations of substitutions until I got it and this doesn't seem very mathematical. Any other ideas? Thanks
Try expanding and then notice that

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