\(Bài.4:\\ a,5\sqrt{25a^2}=5\sqrt{\left(5a\right)^2}=5.\left|5a\right|=5.\left(-5a\right)=-25a\left(Do:a\le0\right)\\ b,\sqrt{16a^4}+6a^2=\sqrt{\left(\text{4a}^2\right)^2}+6a^2=4a^2+6a^2=10a^2\\ c,\sqrt{49a^2}+3a=\sqrt{\left(7a\right)^2}+3a=\left|7a\right|+3a=7a+3a\left(Do:a\ge0\right)=10a\\ d,3\sqrt{9a^6}-6a^3=3\sqrt{\left(3a^3\right)^2}-6a^3=3\left|3a^3\right|-6a^3\\ =3.\left(-3a^3\right)-6a^3\left(Do:a\le0\right)=-9a^3-6a^3=-15a^3\)
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