Find the value of:
Find the perimeter of the rectangle whose length is \(40\) \(cm\) and a diagonal is \(41\) \(cm\).
In the case of right-angled triangles, identify the right angles.
Find the equation of the curve passing through the point \((0,\frac \pi4) \)whose differential equation is, \(sin\ x cos \ y\ dx+cos\ xsin\ y\ dy=0\)