उत्तर (46)

समाधान

मुख्य अवधारणा, $A-B=R-(A \cap B)$

$R_1=\lbrace (a, b): \quad b$ a से विभाज्य है $\rbrace $

$R_1=\lbrace (1,1), (1,2),(1,3), \ldots, (1,20), \\ (2,2), (2,4)(2,6) , \ldots , (2,20) \\ (3,3),(3,6),(3,9) , \ldots , (3,18) \\ (4,4), (4,8), (4,16), (4,20) \\ (5,5),(5,10),(5,15), (5,20) \\ (6,6),(6,12),(6,18),(7,7),(7,14),(8,8),(8,16),(9,9),(9,10),(10,10),(10,20),(11,11),(12,12),\ldots ,(20,20)\rbrace $

$n\left(R _1\right)=20+10+6+5+4+3+2+2+2+2+\underbrace{1+\ldots+1} _{10 \text { times }}$

$n\left(R _1\right)=66$

$R _1 \cap R _2=\lbrace(1,1),(2,2), \ldots(20,20)\rbrace$

$n\left(R _1 \cap R _2\right)=20$

$n\left(R _1-R _2\right)=n\left(R _1\right)-n\left(R _1 \cap R _2\right)$

$=n\left(R _1\right)-20$

$=66-20$

$R _1-R _2=46$ जोड़े