Unit 2
4 marks · NormalA circle has a diameter of 18 cm.
a) Work out the radius of the circle.
b) The area of the circle is decreased by 10%. Work out the new area of the circle. Give your answer in cm² to 2 decimal places.
Work out the radius of the circle.
The area of the circle is decreased by 10%. Work out the new area of the circle. Give your answer in cm² to 2 decimal places.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
Show answer
(a) 9 cm
(b) 229.02 cm^2
Common mistakes: Forgetting to halve diameter for radius; Using wrong formula for area; Misapplying percentage decrease
Show mark scheme and worked solution
Marking points
B1 (a) · 1 mark · Correctly finds the radius as 9 cm.
M1 (b) · 1 mark · Finds original area using .
M2 (b) · 1 mark · Finds 90% (or multiplies area by 0.9) for decrease. (follow-through)
A1 (b) · 1 mark · Gives final answer correct to 2 decimal places. (follow-through)
Worked solution
a) The radius is half the diameter: 18 cm ÷ 2 = 9 cm.
b) Original area: cm² (to 2 d.p.).
Decrease by 10% = 90% of original: cm² (to 2 d.p.).