Q1) If \(cos(x-y)+cos(y-z)+cos(z-x)=\frac{-3}{2}\) and \(cosx+cosy+cosz=a\) and \(sinx+siny+sinz=b\). Find \(a\) and \(b\)

Q2) Two non-conducting infinite wires have shapes as shown above. The linear charge densities of wire (1) and wire (2) are

\(x\quad if\quad x\leq -a~ and~ x\geq a\\ \\ 0\quad if\quad -a<x<a)\)

and

\(y\quad if\quad y\leq -a~ and~ y\geq a\\ \\ 0\quad if\quad -a<y<a)\)

respectively where \((x,y)\) are coordinates of points on the wire. An electron is released from origin, the unit vector along the direction of velocity of the electron just after its release will be in terms of \(\hat { i } \) and \(\hat { j } \)

## Comments

Sort by:

TopNewest\( cosx + cosy + cosz = a \)

\( \cos^2x + \cos^2y + \cos^2z + 2\cos x\cos y + 2\cos y\cos z + 2 \cos x\cos y = a^2\)

Likewise, \( \sin^2x + \sin^2y + \sin^2z + 2\sin x\sin y + 2\sin y\sin z + 2 \sin x\sin y = b^2\)

Adding, \( \cos^2x + \cos^2y + \cos^2z + 2\cos x\cos y + 2\cos y\cos z + 2 \cos x\cos y + \)

\( \sin^2x + \sin^2y + \sin^2z + 2\sin x\sin y + 2\sin y\sin z + 2 \sin x\sin y = a^2 + b^2 \)

\( \implies 1 + 1 + 1 + 2(\cos x\cos y + \cos y\cos z + \cos x\cos y + \sin x\sin y + \sin y\sin z + \sin x\sin y) = a^2 + b^2 \)

\( \implies 3 + 2(\cos(x-y) + \cos(y-z) + \cos(z-x)) = a^2 + b^2 \)

\( \implies 3 + 2* \frac{-3}{2} = a^2 + b^2 \)

\( 0 = a^2 + b^2 \implies a = 0\quad \& \quad b = 0 \) – Siddhartha Srivastava · 2 years, 3 months ago

Log in to reply

– Raghav Vaidyanathan · 2 years, 3 months ago

Thanks for this man.Log in to reply

The answer to q 1 is very easy: \(a=b=0\). – Raghav Vaidyanathan · 2 years, 3 months ago

Log in to reply

– Raghav Vaidyanathan · 2 years, 3 months ago

Ans 2 q2 seems to be zero too.Log in to reply

– Tanishq Varshney · 2 years, 3 months ago

can u explain in brief the first partLog in to reply

– Mvs Saketh · 2 years, 3 months ago

for first one The simplest way is to assume all the three angle differences equal, which ofcourse then must be 120 degree, so just consider 1 , w ,w^2 , add em upLog in to reply

@Raghav Vaidyanathan @Ronak Agarwal @Mvs Saketh – Tanishq Varshney · 2 years, 3 months ago

Log in to reply