How it works ?
In the applet below,
1) Note vectors u and v shown geometrically. (You can move them around by dragging the LARGE PINK POINTS or by moving the vectors themselves). You can zoom in/out if you need to.
2) Move the LARGE WHITE POINTS and/or drag the THICK BLACK VECTOR around to create a vector that is the resultant = sum of u and v.
3) Select the Check Resultant check box to check your answer.