Question

A triangle and a square are drawn on dotty paper with dots 1 cm apart. What is the area of the region coloured red? ​

Answer 1

Answer:

≈ 0.9 cm²

Step-by-step explanation:

I don't know if there is any easier way to solve this, I use 8th grade slope and line method to solve. suppose A is on origin, then the coordinates of triangle ABC are (0,0) (1,3) (4,1) and polygon DEFGH intersect line BC at F and G.

F is the intersect of y=2 and line BC, while G is intersect of x=3 and line BC.

Line BC: pass (1,3) and slope (m) = (1-3)/(4-1) = - 2/3,    

F(x,2)

(2-3) / (x-1) = -2/3            

2x-2 = 3       x = 2.5          

*F (2.5 , 2)

G (3,y)

(y-3) / (3-1) = -2/3     y = 5/3      

*G (3 , 5/3)

*point I (3 , 2)

segment FI = 3-2.5 = 0.5 = 1/2

segment GI = 2 - 5/3 = 1/3

area ΔFIG = (1/2 * 1/3) / 2 = 1/12 = 0.083

area of polygon  DEFGH (region colored RED) = 1 - 0.083 = 0.917 ≈ 0.9 cm²