Tính nhanh :
a) 252 - 152 = (25 + 15)(25 - 15) = 40 . 10 = 400
b) 872 + 732 - 272 - 132 = (872 - 132) + (732 - 272)
= (87 + 13)(87 - 13) + (73 + 27)(73 - 27)
= 100 . 74 + 100 . 26 = 100 . (74 + 26) = 100 . 100 = 10000
Bài 1:
a)\(\left(x+y\right)^2-\left(x-y\right)^2=\left(x+y+x-y\right)\left(x+y-x+y\right)=2x\cdot2y=2\left(x+y\right)\)
b) \(\left(3x+1\right)^2-\left(x+1\right)^2=\left(3x+1+x+1\right)\left(3x+1-x-1\right)\\ =\left(4x+2\right)\cdot2x=4x\left(2x+1\right)\)
Bài 2:
a) \(25^2-15^2=\left(25-15\right)\left(25+15\right)=10\cdot40=400\)
b) \(87^2+73^2-27^2-13^2=\left(87^2-27^2\right)+\left(73^2-13^2\right)\\ =\left(87-27\right)\left(87+27\right)+\left(73-13\right)\left(73+13\right)\)
\(=60\cdot114+60\cdot86=60\cdot\left(114+86\right)=60\cdot200=12000\)
Bài 2:
a) \(x^3-0,25\cdot x=0\)
\(\Leftrightarrow x^2\left(x-0,25\right)=0\)
\(\Leftrightarrow\left[\begin{array}{nghiempt}x=0\\x-0,25=0\end{array}\right.\)\(\Leftrightarrow\left[\begin{array}{nghiempt}x=0\\x=0,25\end{array}\right.\)
b) \(x^2-10=-25\)
\(\Leftrightarrow x^2=-15\) (vô nghiệm0
c) \(4x^2-4x=-1\)
\(\Leftrightarrow4x^2-4x+1=0\)
\(\Leftrightarrow\left(2x-1\right)^2=0\)
\(\Leftrightarrow2x-1=0\Leftrightarrow x=\frac{1}{2}\)
d) \(8x^3+12x^2+6x+1=0\)
\(\Leftrightarrow\left(2x+1\right)^3=0\)
\(\Leftrightarrow2x+1=0\Leftrightarrow x=-\frac{1}{2}\)
Phân tích thành nhân tử :
a) (x + y)2 - (x - y)2 = [(x + y) + (x - y)][(x + y) - (x - y)]
= 2x . 2y = 4xy
b) (3x + 1)2 - (x + 1)2 = [(3x + 1) + (x + 1)][(3x + 1) - (x - 1)]
= (4x + 2)(2x + 2) = 4 . (2x + 1)(x + 1)
