To complete the square for the quadratic expression \( t^2 - 4t + __ \), we need to find a number to add that will make it a perfect square.
- Take the coefficient of \( t \), which is \( -4 \).
- Divide it by 2: \( \frac{-4}{2} = -2 \).
- Square the result: \( (-2)^2 = 4 \).
Now we can complete the square:
\[ t^2 - 4t + 4 \]
This expression can be rewritten as:
\[ (t - 2)^2 \]
So, the number that you need to fill in is \( 4 \). Thus, the completed expression is:
\[ t^2 - 4t + 4 = (t - 2)^2 \]