Answer:
16
Step-by-step explanation:
Hello, do you remember that result?
For any a and b real numbers,
[tex](a+b)^2=a+2\cdot a \cdot b+b^2[/tex]
In this example, we have.
[tex]u^2+8u=u^2+2\cdot 4 \cdot u\\\\\text{ This is the beginning of ... } u^2+8u+4^2=u^2+8u+16\\\\\text{ So, we need to add 16 to make a perfect square}\\\\u^2+8u+\boxed{16}=u^2+2\cdot 4\cdot u +4^2=(u+4)^2[/tex]
Hope this helps.
Do not hesitate if you need further explanation.
Thank you
