Which of the following is $NOT$ a characteristic feature of the class $Mammalia$?

If $y_k$ is the $k$-th derivative of $y$ with respect to $x$, and $y = \cos(\sin x)$, then $y_1 \sin x + y_2 \cos x$ is equal to

$A$ homogeneous solid cylinder of length $L$ $(L < H/2)$ and cross-sectional area $A'$ is immersed such that it floats with its axis vertical at the liquid-liquid interface with length $L/4$ in the denser liquid (density $2d$) and $3L/4$ in the lighter liquid (density $d$) as shown in the figure. The density $D$ of the solid is given by:

If the image of $\left(\frac{-7}{5}, \frac{-6}{5}\right)$ in a line is $(1, 2)$,then the equation of the line is

To the free end of a spring hanging from a rigid support,a block of mass $m$ is hung and slowly allowed to come to its equilibrium position. Then the stretching in the spring is $d$. If the same block is attached to the same spring and allowed to fall suddenly,the amount of stretching is: (force constant,$k$)

$\int_{-1}^1 \frac{\cosh x}{1+e^{2 x}} d x$ is equal to :