Answer:
10x13 is 130
130x8 is 1040
Since the door is part of the wall, but won't be painted, you subtract 1040-21 which is 1019
Your answer is 18 :)
work:
pi(2)d
pi(72)
72/2=36=d
2r=d
36/2=18=r
Similar steps:
1. You need to draw a reference line first( It's trivial but hey, it's similar)
2. You need to draw the other line with pre-defined slope( parallel with same slope, perpendicular with the product of the slope to be -1)
4x²-5=3x+4
4x²-3x-9=0
x=(3±√((-3)²-4*4*-9))/2*4
x=3±√(9+144) /8
x=(3±√153) /8
x=⅜±√153/8