√-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.