I want use math operations in XAML code but I dont know how?
You mean you want to perform arithmetic operations in xaml?
check out this post (it comes with a sample project)
It allows you to specify code like this:
<TextBlock Height="{Binding ElementName=tb1, Path=ActualHeight,
Converter={StaticResource JScript},
ConverterParameter=Int32.Parse(values[0])/10 *100-110 }" Background="Red"/>