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Questions  

 Consider ΔABC with A(a);B(b) and C(c) . If b(a+c)=bb+ac;|ba|=3;|cb|=4  then 

 the angle between the medians AM and BD is : 

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a
π − cos−1 1513
b
π − cos−1 1135
c
cos−1 1513
d
cos−1 1135

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detailed solution

Correct option is A

b→ ⋅ a→ + b→ ⋅ c→  =  b→ ⋅ b→  +  a→ ⋅ c→ or  b→⋅(a→−b→)−c→⋅(a→−b→)=0 or  (b→−c→)⋅(a→−b→)=0⇒BC and AB are perpendicular.  Now find angle between AM and BD M=4i^2=2i ^   AM=2i^-3j^D=4i^+3j^2   BD=4i^+3j^2If θ is the angle between AM and BD then  cosθ=AM·BDAMBD=8-9213254=-1513


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