Nella figura a fianco sappiamo che y = 4.8.  Quanto vale x?

x / 4.8 = 5 / 3 x = 5 / 3 · 4.8 = 5 · 1.6 = 10 · 0.8 = 8

Le figure sono state realizzate con due semplici script, uno e due

Con WolframAlpha:
4.8*5/3
8
6*3/2
9
polygon(0,6),(0,0),(10,6),(0,0),(9,0),(0,0), circle center (5,3) radius 0.1, circle center (8,4.8) radius 0.1
polygon(0,6),(0,0),(10,6),(0,0),(9,0),(0,0), polygon(0,3),(5,3),(5,0),(5,3), polygon(8,0),(8,4.8),(0,4.8),(8,4.8)

# Come possono essere realizzate con R (vedi):
#
source("http://macosa.dima.unige.it/r.R")
BF=3; HF=2.5
PLANEww(-3,10, -3,7)
line(0,-5, 0,15, 1)
line(-5,0, 15,0, 1)
graph1(f, -4,11, "brown")
POINT(5,3, "black")
POINT(8,f(8), "black")
text(6.5,f(8)+0.3,"(x,y)",cex=0.8)
text(6.5,3-0.3,"(5,3)",cex=0.8)
line(5,3, 5,0, "seagreen")
line(5,3, 0,3, "seagreen")
#
PLANEww(-3,10, -3,7)
line(0,-5, 0,15, 1)
line(-5,0, 15,0, 1)
graph1(f, -4,11, "brown")
POINT(5,3, "black")
POINT(8,f(8), "black")
text(8/2,f(8)+0.7,"x",cex=0.8)
text(9,f(8)/2,"4.8",cex=0.8)
text(5/2,3.7,"5",cex=0.8)
text(5.6,3/2,"3",cex=0.8)
line(5,3, 5,0, "seagreen")
line(5,3, 0,3, "seagreen")
line(8,f(8), 8,0, "seagreen")
line(8,f(8), 0,f(8), "seagreen")