3. For each of the following sets of integers, express their greatest common divisor as a linear combination of these integers
a) $6,10,15$
b) $70,98,105$
c) $280,330,405,490$.
3. a) $1=1 \cdot 6+1 \cdot 10+(-1) 15$
b) $7=0 \cdot 70+(-1) 98+1 \cdot 105$
c) $5=-5 \cdot 280+$
$4 \cdot 330+(-1) 405+1 \cdot 490$