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# AS Level Maths Q HELP watch

1. The sector of a circle with radius r has perimeter 40cm
The sector has area A
Q) Show that A=20r-r²

Help is much appreciated!
2. Hiya, the length of the are of the circle is, C=rθ. The perimeter of the sector would be two times the length of the radius + the length of the arc = 40 (as given). This written as an expression would be, 2r + C = 40 , which would be 2r + rθ = 40. The area of the sector is, A = 1/2 r^2 θ . rearrange the equation of the area to subsitute θ. you should get the expression. θ = 2A/r^2 , which you sub into the other equation to get 2r + 2A/r =40. rearranging this to get the equation above

hope i helped
3. (Original post by AudiRS7)
Hiya, the length of the are of the circle is, C=rθ. The perimeter of the sector would be two times the length of the radius + the length of the arc = 40 (as given). This written as an expression would be, 2r + C = 40 , which would be 2r + rθ = 40. The area of the sector is, A = 1/2 r^2 θ . rearrange the equation of the area to subsitute θ. you should get the expression. θ = 2A/r^2 , which you sub into the other equation to get 2r + 2A/r =40. rearranging this to get the equation above

hope i helped
Thanks a lot

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