#### aiklone1314

http://img211.imageshack.us/img211/6048/a1a1.gif [Broken]
http://img17.imageshack.us/img17/6684/b1b1.gif [Broken]

May i know actually why initially 2SA+SB=l , but after differentiate, it becomes 2 aA = -aB ?
why does the l disappear?

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Matt

#### tiny-tim

Homework Helper
Hi aiklone1314!
May i know actually why initially 2SA+SB=l , but after differentiate, it becomes 2 aA = -aB ?
why does the l disappear?
2SA+SB = l, so 2aA+aB=0 (because l is a constant), so 2aA = -aB

#### aiklone1314

but how to know SA and SB is not a constant?

#### tiny-tim

Homework Helper
but how to know SA and SB is not a constant?
If sA is constant, then vA and aA will be 0 (same for sB)

#### aiklone1314

If sA is constant, then vA and aA will be 0 (same for sB)
ok thank you very much...

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