In a parallelogram, the lengths of the two adjacent sides are represented as \( a \) and \( b \). The length of the diagonal connecting these sides can be expressed as \( \sqrt{a^2 + b^2 + 2ab \cos(\theta)} \), where \( \theta \) is the angle between these two sides. If \( a = 5 \) and \( b = 7 \), find the maximum value of the diagonal length. Let's think step by step and output the final answer within \boxed{}.
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