Lạy lần sau hãy quay ảnh đúng chiều
\(a.\left(x+y\right)^2-4^2\)
19)
a) \(\left(x+y+4\right)\left(x+y-4\right)=\left(x+y\right)^2-16\)
b) \(\left(x+y+6\right)\left(x+y-6\right)=\left(x+y\right)^2-36\)
c) \(\left(y+2z-3\right)\left(y-2z-3\right)=\left(y-3+2z\right)\left(y-3-2z\right)=\left(y-3\right)^2-4z^2\)
d) \(\left(x+2y+3z\right)\left(2y+3z-x\right)=\left(2y+3z+x\right)\left(2y+3z-x\right)=\left(2y+3z\right)^2-x^2\)
20)
a) \(\left(x+1\right)^2-\left(x-1\right)^2-3\left(x-1\right)\left(x+1\right)\)
\(=\left(x^2+2x+1\right)-\left(x^2-2x+1\right)-3\left(x^2-1\right)\)
\(=4x-3x^2+3=-3x^2+4x+3\)
b) \(5\left(x+2\right)\left(x-2\right)-\dfrac{1}{2}\left(6-8x\right)^2+17\)
\(=5\left(x^2-4\right)-\dfrac{1}{2}.4\left(3-4x\right)^2+17\)
\(=5x^2-20-2\left(9-24x+16x^2\right)+17\)
\(=5x^2-20-18+48x-32x^2+17\)
\(=-27x^2+48x-21\)
