A simple math torture game
I think you meant to write max(val_1, val_2) instead of max(val_2, val_2).

Long Strings

I personally think the PEP8's 79 characters-per-line requirement is too strict and is not practically optimal. Increase it to, for instance, 100 for this code snippet and you'll get a more readable and concise code with all of the used strings fitting on a single line. 

Other Improvements


I would apply an "Extract Variable" refactoring method and define a tuple of report template strings, which would allow to bring the nestedness 1 level down: 

is_answer_correct = int(input()) == answer
print(ANSWER_TEMPLATES[is_answer_correct].format(answer))
correct_answers += is_answer_correct


where ANSWER_TEMPLATES is defined as:

ANSWER_TEMPLATES = (
    'No, the correct answer was {:d}',
    'Yes, the answer is {:d}'
)



As far as generating unique random pairs for the problem, there is also this approach involving keeping track of seen pairs and keeping generating a next pair until it has not been seen.