Constructing an angle of 90 degrees

1
An angle of 90 at a given point on a line
Task C is a point on line AB.
Construct an angle of 90 at C.
Task
Task C is a point on line AB.
Construction
Conclusion

• YC is the perpendicular to AB.

• Centre O, radius OC, draw an arc to cut AB again
  at X.

• Join XO and produce it to meet the circle at Y.

• Mark any point O outside AB.

• Join CY.

Y

• ?YCB is 90.

O
X