x( x + 1 )/5 - x/15 = 11/5 - (x - 3 )²/15  mmc = 15

3x ( x + 1 )/15 - x/15 = 33/15 - ( x - 3 ) ²/15

3x² + 3x - x = 33 -( x² - 2.3.x + 3²)

3x² + 2x = 33 -x² + 6x - 9

3x² + x² + 2x - 6x = 33 - 9

4x² - 4x - 24 = 0

Δ = b² - 4.a.c

Δ = 16 + 384

Δ = 400

√Δ =20

x' = -b +√Δ/2a

x' = 4 + 20/2.4

x' = 24/8 = 3

x" = -b - √Δ/2a

x" = 4 - 20/2.4

x" = - 16/8

x" = -2

S = { 3, -2}

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