Question 12***+A student expands \((x-6)^2\) as \(x^2-36\). Explain the mistake and give the correct expansion.
Question 17****+Choose the most useful form of \(x^2-10x+21\) for finding its minimum value, then find that minimum.
Question 19*****A simplification gives \(\frac{x^2-1}{x-1}=x+1\). Explain why this is not equivalent to the original expression for every real \(x\).