87903 If a→,b→,c→ are three vectors, such that a→+b→+c→=0→.|a→|=1,|b→|=2,|c→|=3, then a→.b→+b→.c→+c→.a→ is equal to
(B) :Given,a→+b→+c→=0|a→|=1,|b→|=2 and |c→|=3a→⋅b→+b→⋅c→+c→⋅a→= ?We know that,(a→+b→+c→)=|a→|2+|b→|2+|c→|+2(a→⋅b→+b→⋅c→+b→⋅a→)0=12+22+32+2(a→⋅b→+b→⋅c→+c→⋅a→)2(a→⋅b→+b→⋅c→+c→⋅a→)=−14a→⋅b→+b→⋅c→+c→⋅a→=−7
87904 Let a^ and b^ be two unit vectors such that the angle between them is π4. If θ is the angle between the vectors (a^+b^) and (a^+2b^+2(a^×b^)), then the value of 164cos2θ is equal to :
(A) :Given that angle between a^ and b^π4=ϕa. b^=|a^||b^|cosϕNow, a^⋅b^=cosϕ=12Now, cosθ=(a^+b^)⋅(a^+2b^+2(a^×b^))|a^+b^||a^+2b^+2(a^×b^)|. (i)|a^+b^|2=(a^+b^)⋅(a^+b^)|a^+b^|2=2+2a^⋅b^=2+2Now, a→×b→=|a^||b^|sinϕn^a^×b^=n^2 when n^ is vector perpendicular a^ and b^Let, c→=a^×b^When know, c→×b→=0,c→⋅b→=0|a^+2b^+2c→|2=1+4+(4)2+4a^⋅b^+8b^⋅c^+4a^⋅c^=7+42=7+22=(a^+b^)⋅(a→+2b→+2c→)=|a^|2+2a^⋅b^+0+b^⋅a^+2|b^|2+0=1+22+12+2=3+32By equation (i)Now, cosθ=3+322+27+22cos2θ=9(2+1)22(2+2)(7+22)cos2θ=(922)(2+1)(7+22)164cos2θ=(82)×(9)(2+1)(7−22)2(7+22)(7−22)=822(9)(72−4+7−22)(41)=(92)[52+3]=90+272
87905 Let âa and be two unit vectors such that |(a^+b^)+2(a^×b^)|=2. If θ∈(0,π) is the angle between a^ and b^, then among the statements:(S1):2|a^×b^|=|a^−b^|(S2) : The projection of a^ on (a^+b^) is 12
(C) : Given,|(a^+b^)+2(a^×b^)|=2,θ∈(0,π)[(a^+b^)+2(a^×b^)][(a^+b^)+2(a^×b^)]=4|a^+b^|2+4|(a^×b^)|2+0=4Let the angle be θ between a^ and b^2+2cosθ+4sin2θ=42+2cosθ−4cos2θ=0Let,Then,cosθ=tcosθ=t2t2−t−1=02t2−2t+t−1=02t(t−1)+(t−1)=0(2t+1)(t−1)=0t=−12 or t=1cosθ=−12θ=2π3An t=1 not possible as θ∈(0,π).Now,S1=2|a→×b→|=2sin(2π3)|a→−b→|=1+1−2cos(2π3)=1+1−2×(−12)=3Hence, S1 is correct.S2 projection of a^ on (a^+b^)a^⋅(a^+b^)|a^+b^|=1+cos(2π3)2+2cos2π3=1−121=12Hence, S2 is also correct.
87906 Let a^,b^ be unit vectors. If c^ be a vector such that the angle between a^ and c→ is π12, and b^=c→+2(c→×a^) then |6|2 is equal to
(C) : We have,|b^|2=|c→+2(c→×a^)|2|b→|2=|c→|2+4|c→×a^|2+4c→(c→×a^)1=|c→|+4|c→|2sin2π12+01=|c→|2+4|c→|2(3−122)2|c→|2=13−3=3+36Hence, 62|c→|2=6(3+3)