1. First, we set the language bar as the pinyin input method (as shown below):

2. Then click the small triangle (option) next to the word "Zhong", select "Soft Keyboard" in the drop-down menu that appears, and click (as shown below):

3. After clicking "Soft Keyboard", a drop-down menu will appear. At this point, we select the "Mathematical Symbol" with the symbol "+-×" and click it (as shown in the figure below):

4. At this time, there will be a keypad with symbols of addition, subtraction, multiplication and division (+-×) (as shown below):

Finally, we can enter the symbol of addition, subtraction, multiplication and division (+-×) in the text box you want to enter (as shown below):