Bonjour,
7)
25x²-9=0 <=> (5x+3)(5x-3)=0
<=> 5x+3=0 ou 5x-3=0
<=> 5x=-3 ou 5x=3
<=> x=-3/5 ou x=3/5
8)
x²=16 <=> x=√16 ou x=-√16
<=> x=4 ou x=-4
9)
x²+3x=0 <=> x(3+x)=0
<=> x=0 ou 3+x=0
<=> x=0 ou x=-3
10)
3(2x+5)=0 <=> 6x+15=0
<=> 6x=-15
<=> x=-15/6
<=> x=-5/2