Evaluate ab if a+b=13, a-b=5

Anonymous has provided the correct answer.....let's enumerate a little further...

We have

a + b = 13

a - b =    5        add these equations together

2a = 18            divide both sides by 2

a = 9                and using a + b = 13, we have

9 + b = 13        subtract 9 from both sides

b = 4

And there you go......

Very easy. a=9 and b=4