Using (x+a)(x+b)=$x^2$+(a+b)x+ab,find the value of

$\left(a\right)103\times 104$

$\left(b\right)103\times 98$

$\left(a\right)103\times 104=(100+3)\times (100+4)$

$={100}^2+(3+4)\times 100+3\times 4$

$=10000+700+12=10,712$

$\left(b\right)103\times 98=(100+3)\times (100-2)$

$={100}^2+(3-2)\times 100+3\times -2$

$=10,000+100-6=10,094$