$a) (2x + 3)^{2}$ + $(2x - 3)^{2}$ - $(2x + 3)^{}$ $(4x – 6)^{}$+ $xy^{}$
= $ (2x + 3)^{2}$ + $(2x - 3)^{2}$ - $(2x + 3)^{}$ $2(2x – 3)^{}$+ $xy^{}$
= $ (2x + 3)^{2}$ + $(2x - 3)^{2}$ - $(2x + 3)^{}$ $(2x – 3)^{}$-$(2x + 3)^{}$ $(2x – 3)^{}$+ $xy^{}$
= $ (2x + 3)^{}$[$ (2x + 3)^{}$-$(2x – 3)^{}$]+$ (2x - 3)^{}$[$ (2x - 3)^{}$-$(2x + 3)^{}$]+ $xy^{}$
= [$ (2x + 3)^{}$-$(2x – 3)^{}$][$ (2x + 3)^{}$-$ (2x - 3)^{}$]+ $xy^{}$
= $[ (2x + 3)^{}$-$(2x – 3)^{}]^{2}$+ $xy^{}$ (1)
Thay x = 2 ; y = -1 vào (1) ta được :
$[ (2x + 3)^{}$-$(2x – 3)^{}]^{2}$+ $xy^{}$
=$[ (2.2 + 3)^{}$-$(2.2 – 3)^{}]^{2}$+ $2.(-1)^{}$
=$ 6^{2}$-$1^{2}$-$2^{}$
= $36^{}$ -$2^{}$
=$34^{}$
b) $(x – 2)^{2} – (x – 1)(x + 1) – x(1 – x)^{}$
= $(x – 2)^{2} – (x – 1)(x + 1) + x(x - 1)^{}$
= $(x – 2)^{2} – (x – 1)(x + 1) + x^{2}+x-1^{}$
= $(x – 2)^{2} – (x – 1)(x + 1) + (x^{2}-1)+x-1^{}$
= $(x – 2)^{2} – (x – 1)(x + 1) + (x-1)(x+1)+x-1^{}$
= $(x – 2)^{2}$ +$x^{}$-$1^{}$ (1)
Thay x = -2 vào (1) ta được
$(x – 2)^{2}$+$x^{}$-$1^{}$
= $(-2– 2)^{2}$ +$(-2)^{}$-$1^{}$
= $(-4)^{2}$ -$2^{}$-$1^{}$
= 16 -$2^{}$-$1^{}$
= 19
CHÚC BẠN HỌC TỐT !
NHỚ CHO MK CTLHN NHA :33
