If A+B+C=180 then prove that

Cos3Acos3B+cos3Bcos3C+cos3Ccos3A=1

Super easy just create a two way frequency table to answer my question im begging someone. a restaurant did a survey among 100 customers to find their food preferences. the customers were asked about their preferences for pasta or rice. out of the total 30 people who liked pasta, 10 liked rice also. there were 40 people who liked rice. ok so all i need is someone to fill these in: like pasta and rice: like pasta, do not like rice: do not like pasta, like rice: do not like pasta or rice: