I didn't manage to solve the question and didn't understand the explanation at the back of the book.

I didn't understand why x + y = 10, 10 is the minimum possible value of x + y and also why the maximum value has the be found when x = y.

Thanks

Here are some examples of x^2 + y^2 = 100

x^2 = 0, y^2 = 10
x = 0, y = 10, x + y = 10


x^2 = 36, y^2 = 64
x = 6, y = 8, x + y = 14


x^2 = 50, y^2 = 50
x is slightly larger than 7, y is slightly larger than 7
x + y is slightly larger than 14


x^2 = 64, y^2 = 36
x = 8, y = 6, x + y = 14


so, the correct answer is choice C

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