First you make the dissimilar fractions into simmilar fractions by finding their LCD
Example: 2/4 and 1\2... Their LCD is 4
next you divide the LCD to the denominator of the fraction and multiply it to tye numerator so the answer will be 2/4 plus 2/4 add them and get the answer of 4/8 or 1/2 
Remember: The LCD of the fractions will be the denominator of the answer
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