How do I get the arc of the circumference in the exercise?

The name of the pictureThe name of the pictureThe name of the pictureClash Royale CLAN TAG#URR8PPP











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I have this exercise:
enter image description here



So my development was:



Draw a line $OB$, that measures $r$ = radius.



The line $OD$ also measures $r$



If the arc $BC = 100$, so the arc $BW = 80$



Then, the triangle $ODB$ is isosceles of $BD$ base and sides of $r$



Since, the arc $BW = 80$ and $angleBDW$ is the center angle, so $angleBDO = 80$, besides the triangle $ODB$ is isosceles, then $angleOBD = 80$, so the center angle $angleDOB = 20$, therefore the arc $BD = 20$.



In a picture:



enter image description here



But according to the guide the result must be $40$, ¿What is wrong with my development?




geometry




share|cite|improve this question





















  • I see nothing wrong with your argument. It is probably just an error or misprint in the guide.
    – John Wayland Bales
    Jul 28 at 21:39










  • By the way, WBD= 80 is not correct.
    – Seyed
    Jul 28 at 21:50











  • @Seyed, was $angleOBD$, edited.
    – Mattiu
    Jul 28 at 23:23










  • That is a very badly drawn picture. It looks like the center of arc BD is W, not O.
    – marty cohen
    Jul 28 at 23:51










  • Yes it looks, but i need know why my answer is wrong
    – Mattiu
    Jul 29 at 1:19














up vote
0
down vote

favorite












I have this exercise:
enter image description here



So my development was:



Draw a line $OB$, that measures $r$ = radius.



The line $OD$ also measures $r$



If the arc $BC = 100$, so the arc $BW = 80$



Then, the triangle $ODB$ is isosceles of $BD$ base and sides of $r$



Since, the arc $BW = 80$ and $angleBDW$ is the center angle, so $angleBDO = 80$, besides the triangle $ODB$ is isosceles, then $angleOBD = 80$, so the center angle $angleDOB = 20$, therefore the arc $BD = 20$.



In a picture:



enter image description here



But according to the guide the result must be $40$, ¿What is wrong with my development?




geometry




share|cite|improve this question





















  • I see nothing wrong with your argument. It is probably just an error or misprint in the guide.
    – John Wayland Bales
    Jul 28 at 21:39










  • By the way, WBD= 80 is not correct.
    – Seyed
    Jul 28 at 21:50











  • @Seyed, was $angleOBD$, edited.
    – Mattiu
    Jul 28 at 23:23










  • That is a very badly drawn picture. It looks like the center of arc BD is W, not O.
    – marty cohen
    Jul 28 at 23:51










  • Yes it looks, but i need know why my answer is wrong
    – Mattiu
    Jul 29 at 1:19












up vote
0
down vote

favorite









up vote
0
down vote

favorite











I have this exercise:
enter image description here



So my development was:



Draw a line $OB$, that measures $r$ = radius.



The line $OD$ also measures $r$



If the arc $BC = 100$, so the arc $BW = 80$



Then, the triangle $ODB$ is isosceles of $BD$ base and sides of $r$



Since, the arc $BW = 80$ and $angleBDW$ is the center angle, so $angleBDO = 80$, besides the triangle $ODB$ is isosceles, then $angleOBD = 80$, so the center angle $angleDOB = 20$, therefore the arc $BD = 20$.



In a picture:



enter image description here



But according to the guide the result must be $40$, ¿What is wrong with my development?




geometry




share|cite|improve this question













I have this exercise:
enter image description here



So my development was:



Draw a line $OB$, that measures $r$ = radius.



The line $OD$ also measures $r$



If the arc $BC = 100$, so the arc $BW = 80$



Then, the triangle $ODB$ is isosceles of $BD$ base and sides of $r$



Since, the arc $BW = 80$ and $angleBDW$ is the center angle, so $angleBDO = 80$, besides the triangle $ODB$ is isosceles, then $angleOBD = 80$, so the center angle $angleDOB = 20$, therefore the arc $BD = 20$.



In a picture:



enter image description here



But according to the guide the result must be $40$, ¿What is wrong with my development?





geometry





share|cite|improve this question












share|cite|improve this question




share|cite|improve this question








edited Jul 28 at 23:22
























asked Jul 28 at 21:26









Mattiu

759316




759316











  • I see nothing wrong with your argument. It is probably just an error or misprint in the guide.
    – John Wayland Bales
    Jul 28 at 21:39










  • By the way, WBD= 80 is not correct.
    – Seyed
    Jul 28 at 21:50











  • @Seyed, was $angleOBD$, edited.
    – Mattiu
    Jul 28 at 23:23










  • That is a very badly drawn picture. It looks like the center of arc BD is W, not O.
    – marty cohen
    Jul 28 at 23:51










  • Yes it looks, but i need know why my answer is wrong
    – Mattiu
    Jul 29 at 1:19
















  • I see nothing wrong with your argument. It is probably just an error or misprint in the guide.
    – John Wayland Bales
    Jul 28 at 21:39










  • By the way, WBD= 80 is not correct.
    – Seyed
    Jul 28 at 21:50











  • @Seyed, was $angleOBD$, edited.
    – Mattiu
    Jul 28 at 23:23










  • That is a very badly drawn picture. It looks like the center of arc BD is W, not O.
    – marty cohen
    Jul 28 at 23:51










  • Yes it looks, but i need know why my answer is wrong
    – Mattiu
    Jul 29 at 1:19















I see nothing wrong with your argument. It is probably just an error or misprint in the guide.
– John Wayland Bales
Jul 28 at 21:39




I see nothing wrong with your argument. It is probably just an error or misprint in the guide.
– John Wayland Bales
Jul 28 at 21:39












By the way, WBD= 80 is not correct.
– Seyed
Jul 28 at 21:50





By the way, WBD= 80 is not correct.
– Seyed
Jul 28 at 21:50













@Seyed, was $angleOBD$, edited.
– Mattiu
Jul 28 at 23:23




@Seyed, was $angleOBD$, edited.
– Mattiu
Jul 28 at 23:23












That is a very badly drawn picture. It looks like the center of arc BD is W, not O.
– marty cohen
Jul 28 at 23:51




That is a very badly drawn picture. It looks like the center of arc BD is W, not O.
– marty cohen
Jul 28 at 23:51












Yes it looks, but i need know why my answer is wrong
– Mattiu
Jul 29 at 1:19




Yes it looks, but i need know why my answer is wrong
– Mattiu
Jul 29 at 1:19















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