Question

Chứng minh các đẳng thức sau:( vế trái = vế phải )

1. ( a + b ) mũ 2 = ( a - b ) mũ 2 + 4ab

2. a mũ 4 - b mũ 4 = ( a - b ) ( a + b ) ( a mũ 2 + b mũ 2 )

3. ( a mũ 2 + b mũ 2 ) ( x mũ 2 + y mũ 2 ) = ( ax - by ) mũ 2 + ( bx + ay ) mũ 2

Answer 1

1. \(\left(a+b\right)^2=\left(a-b\right)^2+4ab\)

\(VP=a^2-2ab+b^2+4ab=a^2+2ab+b^2=\left(a+b\right)^2\)

\(\Rightarrow VT=VP\)

2. \(a^4-b^4=\left(a-b\right)\left(a+b\right)\left(a^2+b^2\right)\)

\(VP=\left(a-b\right)\left(a+b\right)\left(a^2+b^2\right)=\left(a^2-b^2\right)\left(a^2+b^2\right)=a^4+a^2b^2-b^2a^2-b^4=a^4-b^4\)

\(\Rightarrow VT=VP\)

3. \(\left(a^2+b^2\right)\left(x^2+y^2\right)=\left(ax-by\right)^2+\left(bx+ay\right)^2\)

\(VT=\left(a^2+b^2\right)\left(x^2+y^2\right)=a^2x^2+a^2y^2+b^2x^2+b^2y^2\)

\(VP=\left(ax-by\right)^2+\left(bx+ay\right)^2=a^2x^2-2axby+b^2y^2+b^2x^2+2bxay+a^2y^2=a^2x^2+a^2y^2+b^2x^2+b^2y^2\)

\(\Rightarrow VT=VP\)