Hi, my name is Dr. Michael Winter from Brock University, and I’ll be answering the question “How do you know calculators get the right answer?”
This question actually can have a short and a long answer. The short answer actually is: You don't know for sure. You are trusting engineers and scientists that they have built something that works correctly.
The long answer would start explaining how to handle hard/software correctness formally. There are methods which allow you to specify the intended behavior of software and to mathematically prove that your implementation satisfies that specification. This is a whole research area in Computer Science called Formal Methods.« RETURN TO THE QUESTIONS