So, what is the meaning or usage of the word "But" in the red box？ For there is no significant contrast in the sentence before and after, is it just an emphasis? Thanks in advance~
closed as off-topic by tchrist♦ Sep 8 '16 at 13:42
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The "But" here is equivalent to "At this point, we need to remember... Hence we subsitute...."
It is a way of reminding the reader of a formula that has to be applied there which cannot be ignored for this particular situation in the problem.
Hope that answers your question. Greetings :)