What is the sum of square root of -2 and square root of -18?

√-2 + √-18

√-2 = √(-1*2) =  √-1 * √2 = i√2    Recall square root of -1 = i

√-18 = √(-1*18) =  √-1 * √1`8 = i√18    Recall square root of -1 = i

But √18 = √(9*2) = √9 * √2= 3√2

√-18 = i√18= i(3√2) = 3i√2

√-2 + √-18 = i√2 + 3i√2 = (i + 3i)√2 = 4i√2

So the sum is 4i√2,      where i = √-1

Hope this explains it.