1.

In ` A B C ,=`if `(a+b+c)(a-b+c)=3a c ,`then find `/_Bdot`

Answer» We have `(a + c)^(2) - b^(2) = 3ac`
or `a^(2) + c^(2) - b^(2) = ac`
But `cos B = (a^(2) + c^(2) -b^(2))/(2 ac) = (1)/(2)`
So, `B = (pi)/(3)`


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