ABC are points on a circle, centre O. AO=9cm, OC=9cm and AC=15cm. Find the angle ABC.

Diagrams would be used to help visualise the answer. To find the angle OAC, the cosine rule needs to be used: cosA = (b2 + c2 - a2)/2bc. Therefore, looking at the diagram, (152 + 92 - 92)/2x15x9 = cosA cosA= 5/6, cos-1(5/6) = 33.56 degrees (to 4sf). This is angle OAC. Since AOC is an isoceles triangle, angle OAC and angle OCA are equal, so angle OCA is also 33.56 degrees (to 4sf). Because all angles in a triangle add up to 180 degrees, the final angle AOC can be calculated: 180-(33.56+33.56) = 112.88 degrees. Using circle theorems: the angle at the centre is twice the angle at the circumference. Therefore, 112.88/2 = 56.44 degrees. So angle ABC = 56.44 degrees (to 4sf).

Answered by Imogen P. Maths tutor

3116 Views

See similar Maths GCSE tutors

Related Maths GCSE answers

All answers ▸

Show x^2 + 8x +15 = 0 in the form of (x+b)^2 +c (complete the square) and then solve the equation


Solve 3x^2+7x-13=7 to find x.


Prove that the difference of the square of two consecutive odd numbers is always a multiple of 8. [OCR GCSE June 2017 Paper 5]


Solve 3x^2 + 13x + 14 = 0


We're here to help

contact us iconContact usWhatsapp logoMessage us on Whatsapptelephone icon+44 (0) 203 773 6020
Facebook logoInstagram logoLinkedIn logo
Cookie Preferences