\(\left[f'\left(x\right)\right]^2+f\left(x\right).f''\left(x\right)=15x^4+12x\)
\(\Leftrightarrow\left[f\left(x\right)\right]'.f'\left(x\right)+f\left(x\right).\left[f'\left(x\right)\right]'=15x^4+12x\)
\(\Leftrightarrow\left[f\left(x\right).f'\left(x\right)\right]'=15x^4+12x\)
\(\Rightarrow g'\left(x\right)=15x^4+12x\)
\(g'\left(x\right)=0\) có 2 nghiệm nên g(x) có 2 cực trị