Câu 13: So sánh P và Q P = a + {(a – 3) – [( a + 3) – (- a – 2)]}. Q = [ a + (a + 3)] – [( a + 2) – (a – 2)].

P = a + { ( a – 3 ) – [ ( a + 3 ) – ( – a – 2 ) ] }
= a + [ a – 3 – ( a + 3 + a + 2 ) ]
= a + [ a – 3 – ( 2a + 5 ) ]
= a + ( a – 3 -2a – 5 )
= a + ( -8 – a ) 
= a – 8 – a
= -8

Q = [ a – ( a + 3)] – [ ( a + 2 )-(a-2)]
= ( a – a + 3 ) – ( a + 2 -a+2 )
= 3  – 4
=-1

Ta có: -8 < -1 ⇒ P < Q