2/
\(a,20x^2-\left(5x-4\right)\left(5x+4\right)\\ =20x^2-\left[\left(5x\right)^2-4^2\right]\\ =20x^2-\left(25x^2-16\right)\\ =20x^2-25x^2+16\\ =16-5x^2\\ b,\left(x+3\right)^2-\left(x-3\right)^2\\ =\left(x+3-x+3\right)\left(x+3+x-3\right)\\ =6\cdot2x\\ =12x\\ c,\left(x-y\right)^2+x\left(x+2y\right)\\ =\left(x^2-2xy+y^2\right)+x^2+2xy\\ =x^2-2xy+y^2+x^2+2xy\\ =2x^2+y^2\\ d,\left(x-10\right)^2-x\left(x+80\right)\\ =\left(x^2-20x+100\right)-x^2-80x\\ =x^2-20x+100-x^2-80x\\ =-100x+100\)
Bài 3:
a: \(305\cdot295=\left(300-5\right)\left(300+5\right)\)
\(=300^2-25=90000-25=89975\)
b: \(170\cdot230=\left(200-30\right)\left(200+30\right)\)
\(=200^2-30^2=40000-900=39100\)
c: \(\left(10,5\right)^2=\left(10+0,5\right)^2\)
\(=10^2+2\cdot10\cdot0,5+0,5^2\)
=100+10+0,25
=110,25
d: \(104^2-208\cdot24+24^2\)
\(=104^2-2\cdot104\cdot24+24^2\)
\(=\left(104-24\right)^2=80^2=6400\)
3) a) \(305.295=\left(300+5\right)\left(300-5\right)=300^2-5^2=90000-25=8975\)
b) \(170.230=\left(200-30\right)\left(200+30\right)=200^2-30^2=40000-900=39100\)
c) \(\left(10,5\right)^2=\left(10+0,5\right)^2=10^2+2.0,5.10+0,5^2=100+10+0,25=110,25\)
d) \(104^2-208.24+24^2=104^2-2.104.24+24^2=\left(104-24\right)^2=80^2=6400\)