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If |veca|=2, |vecb|=5 and |vecaxxvecb|=8...

If `|veca|=2, |vecb|=5 and |vecaxxvecb|=8` then find the value of `veca.vecb`

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`veca.vecb= |veca||vecb|costheta,but |vecaxxvecb|=|veca||vecb|sintheta`
` or sin theta (|veca xx vecb|)/(|veca||vecb|)= 4/5 Rightarrow 4/5 Rightarrow theta= 3/5`
thereforem , `veca.vecb= 2xx5xx 3/5 = 6`
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