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Questions  

 In the isosceles triangle ABC,_|AB|=|BC|=8, a point E divides AB internally in  the ratio 1:3, then the cosine of the angle between CE and CA is : (where |CA|=12 ) 

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a
−378
b
3817
c
378
d
−3817

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detailed solution

Correct option is C

Given |b→|=|b→−c→|=8 and |c→|=12|b→|2=|b→−c→|2 |b→|2=b→2+c→2-2b→·c→ b→·c→=1442=72c→-b→42=c→2-2c→·b→4+b→216 =144-724+6416=112 c→-b→4=112CE→=OE→-OC→=b→4-c→ CA→=-c→cosθ  =  c→ c→ −b→4|c→ | c→ − b→4     =  c2→ −  c→ ⋅ b→412c→ − b→4=144-1812112=12612×47=378


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