87749 If the points with position vectors −j^−k^,4i^+5j^+λk^,3i^+9j^+4k^ and −4i^+4j^+4k^ are coplanar, then λ equals
(C) : Given,Position vector is :- a→,b→,c→−j^−k^,4i^+5j^+λk^,3i^+9j^+4k^,−4i^+4j^+4k^Points are :-A(0,−1,−1),B(4,5,λ),C(3,9,4),D(−4,4,4) Now,AB→=b→−a→AB→=(4i^+5j^+λk^)−(−j^−k^)AB→=4i^+6j^+(λ+1)k^⇒AC→=c→−a→AC→=(3i^+9j^+4k^)−(−j^−k^)AC→=3i^+10j^+5k^AD→=d→−a→AD→=(−4i^+4j^+4k^)−(−j^−k^)AD→=−4i^+5j^+5k^Given points are coplanar.∴|46λ+13105−455|=04(50−25)−6(15+20)+(λ+1)(15+40)=055λ−55=0λ=1
87750 If the vectors AB→=−3i^+4k^ and AC→=5i^−2j^+4k^ are the sides of a triangle ABC. Then the length of the median through A is
(B) : Given that,AB→=−3i^+4k^AC→=5i^−2j^+4k^We know that,AD→=AB→+AC→2=(−3i^+4k^)+(5i^−2j^+4k^)2|AD→|=2i^−2j^+8k^2=i^−j^+4k^|AD→|=12+12+42=1+1+16∣AD→=18 unit
87751 If |a→|=2,|b→|=3,|c→|=4 and a→+b→+c→=0→, then the value of b→⋅c→+c→⋅a→+a→⋅b→ is equal to
(D) : Given that,|a→|=2,|b→|=3,|c→|=4 and a→+b→+c→=0We known that,|a→+b→+c→|2=(a→+b→+c→)⋅(a→+b→+c→)(0)2=|a→|2+|b→|2+|c→|2+2(a→⋅b→+b→⋅c→+c→⋅a→)(0)2=|2|2+|3|2+|4|2+2(a→⋅b→+b→⋅c→+c→+a→)0=4+9+16+2(a→⋅b→+b→⋅c→+c→⋅a→)∴(a→⋅b→+b→⋅c→+c→⋅a→)=−292
87752 If the vectors i→+3j→−2k→,2i→−j→+4k→ and 3i→+2j→+xk→ are coplanar, then the value of x is:
(B) : Given,a→=i^+3j^−2k^b→=2i^−j^+4k^c→=3i^+2j^+xk^If the vector a→,b→ and c→ are coplanar then[a→b→c→]=0|13−22−1432x|=01(−x−8)−3(2x−12)−2(4+3)=0−x−8−6x+36−14=0−7x+14=0x=2