$\text{a)} (x-7)(x-5)(x-4)(x-2)=72$
$\to [(x-7)(x-2)][(x-5)(x-4)]=72$
$\to (x^2-9x+14)(x^2-9x+20)-72=0$
$\to (x^2-9x+14)[(x^2-9x+14)+6]-72=0$
$\to [(x^2-9x+14)^2+6(x^2-9x+14)+9] -81=0$
$\to (x^2-9x+14+3)^2-81=0$
$\to (x^2-9x+17)-9^2=0$
$\text{Đặt} x^2-9x+17\text{ là t}$
$\to (t-9)(t+9)=0$
$\text{Nếu t-9=0}$
$\to x^2-9x+17-9=0$
$\to x^2-9x+8=0$
$\to x^2-8x-x+8=0$
$\to x(x-8)-(x-8)=0$
$\to (x-8)(x-1)=0$
$\to x=1; x=8$
$\text{Nếu t+9=0}$
$\to x^2-9x+17+9=0$
$\to x^2-9x+26=0\to \text{(vô nghiệm)}$
$\text{b),c) Bạn xem hình..:}$
$\text{Sửa lại câu b:}$
$\dfrac{x+25}{75}+{x+30}{70}=\dfrac{x+35}{65}+\dfrac{x+40}{60}$