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Meilleure réponse !
2013-09-30T21:18:50+02:00
Ex4)
Aire totale = 30*20 = 600 m²
1/5 aire totale = 600* 1/5 = 120 m² 
Aire rectangle intérieur = (30-2x)(20-2x)
Aire intérieure < 1/5 * Aire totale
4x²-100x+600 < 120
4x²-100x+480 < 0
delta = 10 000 - 7680 = 2320
Vdelta = 48.16 
x1 = (100 - 48.16) / 8 = 6.48 mètres 

Ex 1)
x²-5x+12 = 8 
x²-5x+4 = 0
delta = 25 - 16 = 9
Vdelta = 3   deux solutions
x1 = (5-3) / 2 = 1  
x2 = (5+3) / 2 = 4  

3x²-8x-5  
forme canonique = a(x-x1)(x-x2)
delta = 64 -60 = 4     donc Vdelta = 2 
x1 = 2     et x2 = 10/3  
factorisation =  3(x-2)(x-10/3)