Tính nhanh nếu có thể mn giúp mik vs hứa vote 5 sao vs tlhn

Đáp án + Giải thích các bước giải:

`a,4*(25-70)`$\\$`=4*25-4*70`$\\$`=100-280`$\\$`=-180`$\\$`b,`$\dfrac{-3}{4}+\dfrac{3}{7}+\dfrac{-2}{8}+\dfrac{4}{9}+\dfrac{4}{7}$$\\$`=`$\dfrac{-3}{4}+\dfrac{-2}{8}+\dfrac{3}{7}+\dfrac{4}{7}+\dfrac{4}{9}$ $\\$`=`$\dfrac{-6}{8}+\dfrac{-2}{8}+\dfrac{7}{7}+\dfrac{4}{9}$ $\\$`=-1+1+4/9`$\\$`=4/9`$\\$`c,3/4+`$\dfrac{-5}{6}$`-11/-12`$\\$`=3/4+`$\dfrac{-5}{6}$`-`$\dfrac{-11}{12}$ $\\$`=9/12+`$\dfrac{-10}{12}$`-`$\dfrac{-11}{12}$ $\\$`=10/12=5/6`