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6.7k views
in Mathematics by (20 points)

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1 Answer

+2 votes
by (10.9k points)
(2a-b)•(a+3b)=-25

=>2a•a-b•a+6a•b-3b•b-25

=>2|a||a|cos0+5|b||a|cos90-3|b||bcos0=-25

=>2+0-3|b|^2=-25        ( as |a|=1)

=>3|b|^2=27

=>|b|^2=9

=>|b|=3

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