The sixth grade math exercise book volume 2 9 1 page calculates the shadow area below, how to do it! ! Urgent! ! !
Red area+black area = 1/4 circle (the radius of this circle is r=4cm).
Black area = 1/4 circle (the radius of this circle is r = 4cm)- red area.
1/4 circular area = 1/4π*4*4=4π.
Red area: 1/2πr2=2π.
Black area =4π-2π=2π