What is a gcf? and what is a gcf for

Plzzz maths ! quadratic sequences=an^2+c 1) 11,17,27,41,59 2) 7,16,31,52,79 3) 6,18,38,66,102 4) 0,9,24,45,72 5) 0,12,32,60,96

What is the missing constant term in the perfect square that starts with x^2 -20x

[tex]5 \times1 \frac{12}{13} [/tex] me