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    				| 1. | If `(x , y)`and `(x ,y)`are the coordinates of the same point referred to two sets ofrectangular axes with the same origin and it `u x+v y ,`where `u`and `v`are independent of `xa n dy`, becomes `V X+U Y ,`show that `u^2+v^2=U^2+V^2dot` | 
| Answer» Let the axes rotate at angle `theta`. If `(x,y)` is the point with respect to the old axes and (x,y) are the coordinates with respect to the new axes, then `{(x,=,Xcostheta-Ysintheta,,),(y,=,Xsintheta+Ycostheta,,):}` Then `ux+vy=u(Xcostheta-Ysintheta)+v(Xsintheta + Y costheta` `=(ucostheta +vsintheta )X+(-usintheta +vcos theta )Y` But given new exression is `VX+UY`. Then, `VX+UY=(ucostheta +vsintheta )X+(-usintheta +vcos theta) Y` On comparing the coefficients of X and Y, we get `ucostheta +vsintheta=V` (1) and `-usintheta +vcostheta=U` (2) Squareing and adding (1) and (2) , we get `u^2+v^2=U^2+V^2` | |