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.)
