Question

Answer 1

a: Thay x=2/3 vào A, ta được:

\(A=9\cdot\left(\dfrac{2}{3}\right)^2-4=9\cdot\dfrac{4}{9}-4=4-4=0\)

b: \(B=x^2+8x+16=x^2+2\cdot x\cdot4+4^2=\left(x+4\right)^2\)

Khi x=26 thì \(B=\left(26+4\right)^2=30^2=900\)

c: \(C=2x^2-12xy+18y^2=2\left(x^2-6xy+9y^2\right)=2\left(x-3y\right)^2\)

Thay x=39;y=12 thì \(C=2\left(39-3\cdot12\right)^2\)

\(=2\left(39-36\right)^2=2\cdot3^2=18\)

d: \(D=\left(x+y\right)^2=\left(x-y\right)^2+4xy\)

\(=10^2+4\cdot24=100+96=196\)