Which of the following reactions will $NOT$ occur with heated copper at $300\,^\circ C$?

If $dx + dy = (x + y)(dx - dy)$,then $\log(x + y)$ is equal to

Product $(A)$ is

$A$ bi-convex lens is formed with two thin plano-convex lenses as shown in the figure. The refractive index of the first lens is $1.5$ and that of the second lens is $1.2$. Both the curved surfaces have the same radius of curvature $R = 14\; cm$. For this bi-convex lens,if the object distance is $40\; cm$,the image distance will be: (in $; cm$)

$\operatorname{cosec} 15^{\circ} + \sec 15^{\circ}$ is equal to :

The blocks shown in the figure move with a constant velocity of $10 \, m/s$ towards the right. All surfaces in contact are rough. The friction force applied by block $B$ on block $A$ is ........... $N$.