Question

X+y=c
x−y=a

solve for x and y

Answer 1

x = (c+a)/2

y = (c-a)/2

Explanation:

Add the two equations and you get ...

(x +y) +(x -y) = (c) +(a)

2x = c+a

x = (c+a)/2

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Subtract the second equation from the first and you get ...

(x +y) -(x -y) = (c) -(a)

2y = c-a

y = (c-a)/2

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Comment on this problem

This is what I call a "sum and difference" problem. The solution shown here is (obviously) the generic solution to such a problem. Remembering this can get you quickly to the answer any time you're given the sum of two unknowns and their difference.