Question 754482

 An arc is a part of the circumference between two points or a continuous piece of a circle. 

if  arc {{{XY}}} gives you to angle that equal {{{90}}} degrees which is {{{1/4}}} of {{{360}}} degrees that a full circle has, then the measure of arc {{{XY}}} will be {{{XY=2r*pi/4}}} => {{{XY=cross(2)r*pi/cross(4)2}}} => {{{XY=r*pi/2}}}