Write a polynomial of least degree with roots 1 and -2 .

Step 1 ：A polynomial of least degree with roots 1 and -2 can be found by using the fact that if r is a root of a polynomial, then \((x-r)\) is a factor of that polynomial.

Step 2 ：Therefore, the polynomial with roots 1 and -2 is \((x-1)(x+2)\).

Step 3 ：We can multiply these factors to get the polynomial in standard form.

Step 4 ：The polynomial in standard form is \(x^2 + x - 2\).

Step 5 ：Final Answer: The polynomial of least degree with roots 1 and -2 is \(\boxed{x^2 + x - 2}\).