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{b=4/a
4/a +7a =29
4/a+7a-29=0 aducem la acelasi numitor cu a
4+7a² -29a²=0
7a²-29a +4=0
a=7
b= -29
c=4
Δ= (-29)² -4*7*4= 841 -112=729
x1;x2= (29 +/- √729)/4*7= (29 +/- 27)/ 28
x1= (29+27)/28= 56/28=2
x2= (29-27)/28= 2/28= 1/14
7a²-29a+4= (x-2)(x-1/14)= (x-2)(x-1/2)= (x-2)(2x-1)
(x-2)=0 ∧ 2x-1=0
x-2=0 2x=1
x=2 x= 1/2 ∉ N =>x=2 ,=>b=2 =>a=4/2=2