p4. For how many three-digit multiples of 11 in the form abc does the quadratic ax2+bx+c have real roots?
p5. William draws a triangle △ABC with AB=3, BC=1, and AC=2 on a piece of paper and cuts out △ABC. Let the angle bisector of ∠ABC meet AC at point D. He folds △ABD over BD. Denote the new location of point A as A′. After William folds △A′CD over CD, what area of the resulting figure is covered by three layers of paper?
p6. Compute (1)(2)(3)+(2)(3)(4)+...+(18)(19)(20).
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