\(D=50^2-49.51\)
\(\Leftrightarrow D=50^2-\left(50-1\right)\left(50+1\right)\)
\(\Leftrightarrow D=50^2-50^2+1=1\)
\(C=39^2+78.61+61^2\)
\(\Leftrightarrow C=39^2+2.39.61+61^2\)
\(\Leftrightarrow C=\left(39+61\right)^2=100^2=10000\)
a) (7x + 4) 2 - (7x + 4)(7x - 4)
\(=\left(7x+4\right)\left(7x+4-7x+4\right)\)
\(=8\left(7x+4\right)\)
\(=56x+32\)
b)(x +2y)3 -6xy(x + 2y);
\(=\left(x+2y\right)\left(\left(x+2y\right)^2-6xy\right)\)
\(=\left(x+2y\right)\left(x^2+4xy+4y^2-6xy\right)\)
\(=\left(x+2y\right)\left(x^2-2xy+4y^2\right)\)
\(=x^3+8y^3\)
c) (3x + y) (9x2 - 3xy + y2 ) - ( 3y - y)3 - 27x2y
\(=27x^3+y^3-\left(27x^3-27x^2y+9xy^2-y^3\right)-27x^2y\)
\(=27x^3+y^3-27x^3+27x^2y-9xy^2+y^3-27x^2y\)
\(=2y^3-9xy^2\)
