b1:tìm x 1,($$\frac{1}{24.25}$$ + $$\frac{1}{25.26}$$ +...+ $$\frac{1}{29.30}$$ ).120+x= $$\frac{1}{3}$$ 3, $$\frac{3}{(x-4).(x-7)}$$ + $$\frac{6}{(x-7).(x-13)}$$ + $$\frac{15}{(x-13).(x-28)}$$ - $$\frac{1}{x-18}$$ = $$\frac{-5}{2}$$