Find two numbers, if their sum is −11 and their difference is 41

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.

since both sides equal, there are infinitely many olutions.