Bài 1:
a, 78.(-23)+37.(-78)+40.(-78)
=78.(-23-37-40)
=78.(-100)
=-7800
b, $-1\frac{5}{7}.15+$ $\frac{2}{7}.(-15)+(-105)($ $\frac{2}{3}-$ $\frac{4}{5}+$ $\frac{1}{7})$ $=\frac{-12}{7}.(-15)+$ $\frac{-30}{7}+(-105)$ $\frac{1}{105}$ $=\frac{180}{7}+$ $\frac{-30}{7}+(-1)$ $=\frac{150}{7}-1$ $=\frac{143}{7}$
c, $\frac{5.7^{11}+7^{12}}{7^9.5^2-13.7^9}=$ $\frac{7^{11}.(5+7)}{7^9.(5^2-13)}=$ $\frac{7^2.12}{12}=49$
