\(a,\left(7x+2\right)^2-\left(7x-5\right)^2\)
\(=\left[7x+2-\left(7x-5\right)\right]\left(7x+2+7x-5\right)\)
\(=7\left(14x-3\right)\)
Với \(x=-5\), ta được:
\(7.\left[14.\left(-5\right)-3\right]\)
\(=7.\left(-70-3\right)\)
\(=7.\left(-73\right)=-511\)
\(c,\left(x+9\right)^2-\left(x-8\right)^2-\left(32x-1\right)\)
\(=\left(x+9-x+8\right)\left(x+9+x-8\right)-32x+1\)
\(=17.\left(2x+1\right)-32x+1\)
\(=34x+17-32x+1\)
\(=2x+18\)
Với \(x=\dfrac{1}{2}\), ta được:
\(2.\dfrac{1}{2}+18=1+18=19\)
\(a,\left(7x+2\right)^2-\left(7x-5\right)^2\\ =\left[7\cdot\left(-5\right)+2\right]^2-\left[7\cdot\left(-5\right)-5\right]^2\\ =\left(2-35\right)^2-\left(-35-5\right)^2\\ =\left(35-2\right)^2-\left(35+5\right)^2\\ =\left(35-2-35-5\right)\left(35-2+35+5\right)\\ =-7\cdot73\\ =-511\\ b,\left(x+9\right)^2-\left(x-8\right)^2-\left(32x-1\right)\\ =\left(\dfrac{1}{2}+9\right)^2-\left(\dfrac{1}{2}-8\right)^2-\left(32\cdot\dfrac{1}{2}-1\right)\\ =\dfrac{361}{4}-\dfrac{225}{4}-15\\ =34-15\\ =19\)
