To calculate \( 7.7 \times 10^{-4} - 4.9 \times 10^{-5} \), we need a common exponent for the terms. We can rewrite \( 4.9 \times 10^{-5} \) to have the same exponent as \( 7.7 \times 10^{-4} \).
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Convert \( 4.9 \times 10^{-5} \) to the same exponent as \( 10^{-4} \): \[ 4.9 \times 10^{-5} = 4.9 \times 10^{-5} \times \frac{10^{-4}}{10^{-4}} = 4.9 \times 10^{-5} \times 10^{-1} = 0.49 \times 10^{-4} \]
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Now we can subtract: \[ 7.7 \times 10^{-4} - 0.49 \times 10^{-4} = (7.7 - 0.49) \times 10^{-4} = 7.21 \times 10^{-4} \]
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The result in scientific notation is: \[ 7.21 \times 10^{-4} \]
Thus, the answer is \(\boxed{7.21 \times 10^{-4}}\).