Bài 5:
1) \(\left(5+7\right)\left(7-5\right)=7^2-5^2\)
2) \(\left(x+y\right)\left(y-x\right)=y^2-x^2\)
3) \(\left(x-y\right)\left(-x-y\right)=-\left(x+y\right)\left(x-y\right)=-\left(x^2-y^2\right)=y^2-x^2\)
6) \(\left(2+3x^2\right)\left(3x^2-2\right)=9x^4-4\)
7) \(\left(\dfrac{1}{2}+x\right)\left(-x+\dfrac{1}{2}\right)=\left(\dfrac{1}{2}+x\right)\left(\dfrac{1}{2}-x\right)=\dfrac{1}{4}-x^2\)
8) \(\left(4m-5n\right)\left(5n+4m\right)=\left(4m-5n\right)\left(4m+5n\right)=16m^2-25n^2\)
9) \(\left(7a+1\right)\left(1-7a\right)=\left(1+7a\right)\left(1-7a\right)=1-49a^2\)
10) \(\left(1+9\right)\left(1-9\right)=1-9^2\)
Bài 5:
11) \(\left(9a+8b\right)\left(9a-8b\right)=81a^2-64b^2\)
13) \(\left(\dfrac{2}{3}a+\dfrac{4}{5}b\right)\cdot\left(\dfrac{2}{3}a-\dfrac{4}{5}b\right)=\dfrac{4}{9}a^2-\dfrac{16}{25}b^2\)
14) \(\left(\dfrac{3}{2}a^3+\dfrac{2}{3}b^2\right)\left(\dfrac{3}{2}a^3-\dfrac{2}{3}b^2\right)=\dfrac{9}{4}a^6-\dfrac{4}{9}b^4\)
17) \(\left(49x^2-14xy-y^2\right)\left(49x^2-14xy+y^2\right)=\left(49x^2-14xy\right)^2-y^4\)
Bài 6:
1: \(\left(1-2x\right)\left(1+2x\right)=1-4x^2\)
2: \(\left(6x-7y^2\right)\left(6x+7y^2\right)=36x^2-49y^4\)
3: \(\left(3x-5y\right)\left(3x+5y\right)=9x^2-25y^2\)
4: \(\left(6a-9b\right)\left(6a+9b\right)=36a^2-81b^2\)
5: \(\left(\dfrac{2}{3}m-\dfrac{3}{2}n\right)\left(\dfrac{2}{3}m+\dfrac{3}{2}n\right)=\dfrac{4}{9}m^2-\dfrac{9}{4}n^2\)
