a: \(77\cdot13=77\left(10+3\right)=77\cdot10+77\cdot3=770+231=1001\)
\(\left(-77\right)\cdot13=-\left(77\cdot13\right)=-1001\)
\(77\cdot\left(-13\right)=\left(-1\right)\cdot77\cdot13=-1\cdot1001=-1001\)
\(\left(-77\right)\left(-13\right)=77\cdot13=1001\)
b: \(29\cdot\left(-7\right)=\left(-7\right)\left(30-1\right)=-7\cdot30+7=-210+7=-203\)
\(\left(-29\right)\cdot\left(-7\right)=-\left[29\cdot\left(-7\right)\right]=-\left(-203\right)=203\)
\(29\cdot7=-\left[29\cdot\left(-7\right)\right]=-\left(-203\right)=203\)
\(\left(-29\right)\cdot7=\left(-1\right)\cdot7\cdot29=\left(-7\right)\cdot29=-203\)
a) Ta có: $77\cdot13=1001$
Suy ra: $(-77)\cdot13=-1001$ ;
$77\cdot(-13)=-1001$
$(-77)\cdot(-13)=1001$
b) Có: $29\cdot(-7)=-203$
Suy ra: $(-29)\cdot(-7)=203$
$29\cdot7=203$
$(-29)\cdot7=-203$
