1.

In a `DeltaABC," if "/_A=30^(@)"and "b : c =2: sqrt3, " find "/_B`.

Answer» Let b = 2k and c `= sqrt3k`. Then,
`cosA=(b^(2)+c^(2)-a^(2))/(2bc)`
`rArr" "cos30^(@)=(4k^(2)+3k^(2)-a^(2))/(4sqrt3k^(2))`
`rArr" "sqrt3/2xx4sqrt3k^(2)=7k^(2)-a^(2)`
`rArr" "k^(2)=a^(2)rArrk=a.`
Thus, b = 2a and `c=sqrt3a`.
`"Now,"a/("sin A")=b/("sin B")=c/("sin C")=(2a)/("sin B")`
`rArr" "sinB=(2axx1/2xx1/a)=1`
`rArr" "B=90^(@)`.


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