The larger one is (-11 + 41)/2 = 15.
The smaller one is (-11 -41)/2 = -26.
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For two numbers a and b with sum s and difference d, you can write the equations
Then adding the equations and dividing that sum by 2, you get
You can subtract the second eqution from the first and get a similar result for the smaller number
These are the formulas we used above.