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X+3x+5x+...+27x+126=2x+4x+6x+...+28x;

Răspuns :

x+3x+5x+...+27x+126=2x+4x+6x+...+28x
x *(1+3+5+7....+27)+126= 2x *(1+2+3+....14)
x*(27+1)*14/2+126=2x*(14*15/2)
196x+126=210x
210x-196x=126
14x=126
x=126÷14
x=9