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Visitors keep asking about calculator which would compute area of a convex quadrilateral given it's sides. This is mathematically impossible. However, you can find area of a convex quadrilateral, given quadrilateral diagonal values and value of angle between them.
The calculator for the formula is below. After calculator I describe the process of creating a calculator using this site's engine.
To create a calculator, you first should log in into site. Then, in "Personal section" you should choose "Create" -> "Calculator" menu item.
You will get this form
The calculator, basically, is the thing, which receives user input and convert it to output values. So, for our calculator, we need three input parameters: two diagonals and angle. We have to describe them for the engine.
Choose "Input" -> "Add new input parameter" in menu. You will get input parameter editor, which should be filled as on the picture.
Repeat for second parameter
For the third parameter use range feature and limit allowed values with range from 0 to 180 degrees.
Note that each time you change the parameters, signature of function "Calculate" is changed to reflect those.
Now you have to describe output parameter. Choose "Output" -> "New output parameter" and fill the form as below.
Note that input parameters can be used directly, however, output parameter is an object, and the only way to assign value to it, is to use it's special method "SetValue".
Code which calculates area:
Second line: note the usage of SetValue to assign calculation result.
Now you can press "Preview". This button allows you to preview and test the calculations and also saves the calculator source on site.
Now you can publish calculator using button "Publish". After publishing, your calculator will get constant URL on site and will be available for everyone.
The final step is to create article which describes the calculator. Article lets you educate visitors about your calculator, its purpose and details of calculation, and almost always user friendly. To create an article you should choose "Create" -> "Article" in "Personal Section". Then you write some text and insert the calculator into your article, using "Calculator" button on toolbar.