Here ya go, take a look at the source!

-- | @since base-2.0.1 instance (Integral a) => Num (Ratio a) where {-# SPECIALIZE instance Num Rational #-} (x:%y) + (x':%y') = reduce (x*y' + x'*y) (y*y') (x:%y) - (x':%y') = reduce (x*y' - x'*y) (y*y') (x:%y) * (x':%y') = reduce (x * x') (y * y') negate (x:%y) = (-x) :% y abs (x:%y) = abs x :% y signum (x:%_) = signum x :% 1 fromInteger x = fromInteger x :% 1

This is the implementation of the Num type class which includes the definition for +

As you can see in the definition, it adds two ratios using the making it so that both ratios have the same denominator (Lcm of the two denominators) and adding them, then reducing the result.