In adding dissimilar fraction you must find first the least common denominator.. it means that you must find the denominator that can be divided by both denominators. in your example 1/2 + 1/8, the denominators are 2 and 8, you must find a number that can be divided by both. and that is 8.
change 1/2 into a fraction which has the least common denominator. how? divide the LCD by the denominator and multiply the numerator.
8(LCD) divide by 2(denominator of first fraction) = 4
4 multiply by 1(numerator of first fraction) = 4
so it will become 4/8
4/8 + 1/8= 5/8