Length of a chord is 40 cm. If the angle subtended by this chord at the circumference of the circle is 90 degree, then what will be the radius of this circle?

20 cm

Let O be the circle's center, and let C the point on the circumference.

Using, If an angle subtended by a chord by a chord on the circumference of a circle is 90 degrees, then the chord is the circle's diameter.

So, MN is the diameter.

MN = 2(OM)

40 = 2(OM)

OM = 20

Therefore, OM is 20 cm.