This is the question: write down the equation of a circle with centre (0,0) and radius 5. so is my answer correct? x² + y² = 5 √5 = √1 × √5 = 1√5

You almost had it right! remember the equation of a circle is
(x - h)^2 + (y - k)^2 = r^2. the center is (h,k) and r is the radius.

(x - 0)^2 + (y - 0)^2 = 5^2

x^2 + y^2 = 25 <<< your answer

hope that helps, God bless!