bai 2

a) 128-(x+5)=23

=>(x+5)=128-23

=> x+5=105

=>x=100

b (15+x)\(\div\) 3=3¹⁵:3¹²

=>(15+x):3=3³

=> 15+x=3³.3

=> 15+x=81

=> x=81-15=66

c(x+5)-90=235

=> x+5=235+90

=>x+5=325

=>x=325-5=320

b x²+x-6

=x²+3x-2x-6

=x(x+3)-2(x+3)

= (x-2)(x+3)

1

x³-7x+6

=x³+0x²-7x +6

= x³-x²+x²-x-6x+6

=(x³-x²)+(x²-x)-(6x-6)

=x²(x-1)+x(x-1)-6(x-1)

=(x-1)(x²+x-6)

=(x-1)(x²+3x-2x-6)

=(x-1)[x(x+3)-2(x+3)]

=(x-1)(x-2)(x+3)

45² +40²-15²+80.45

=> 45²+80.45+40²-15²

=> (45+40)²-15²

=> 85²-15²

=> (85-15)(85+15)

=> 70.100

=>7000

Ý BẠN LÀ n^3-n hay n³-n

a) (x+2)² -2(x+2)(x-8)+(x-8)²

=[ (x+2)-(x-8)]²

=(x+2-x+8)²

=10²

= 100

VẬY GT CỦA BT K^O PHỤ THUỘC VÀO BIẾN

A=9x²+6x+11

=9x² +6x+1-1+11

=(3x+1)²+10

Do (3x+1)²\(\ge\)0 \(\forall\)x

=>(3x+1)²+10\(\ge\) 10

=>A\(\ge\) 10

GTNN A=10 khi 3x+1=0

=> 3x=-1

=> x=-\(\dfrac{1}{3}\)

N =4x²+4x+1

=(2x+1)²

do (2x+1)2\(\ge\) 0 \(\forall\) x

Min N =0 khi 2x+1=0

=>2x=-1

=>x=-\(\dfrac{1}{2}\)

N=4x2+4x+1

= (2x+1)2

do (2x+1)2 \(\ge0\forall x\)

MinN=0 khi 2x+1=0

=>2x=-1

=>x=\(\dfrac{-1}{2}\)