To subtract #x^2# from #x^2/4#, you need the lowest common denominator. The lowest common denominator (#LCD#) for #4# and #1# is #4#. That is, #4# is the smallest number that both #1# and #4# evenly divide into.
Since #x^2/4# already has #4# as its denominator, that does not need to be changed. The current denominator of #x^2#, however, is #1#, which becomes #4# by multiplying by #4#.
If the denominator is multiplied by #4#, then the numerator must also be multiplied by #4#. Or, #(x^2)(4) = 4x^2#.
Now, subtraction is possible, since the denominators are the same.
And, #x^2/4 - (4x^2)/4 = -(3x^2)/4#.
(Sorry, I answered this as a simplification of the expression. I thought that the "calculus" topic was wrong. Please don't forget to include complete details for future questions that you may ask. Thanks.)
