Trả lời:
a, \(\left(\frac{1}{2}x-3\right)^2=\left(\frac{1}{2}x\right)^2-2.\frac{1}{2}x.3+3^2=\frac{1}{4}x^2-3x+9\)
b, \(\left(2x^2+3y\right)^2=\left(2x^2\right)^2+2.2x^2.3y+\left(3y\right)^2=4x^4+12x^2y+9y^2\)
c, \(\left(3x-7y\right)^2=\left(3x\right)^2-2.3x.7y+\left(7y\right)^2=9x^2-42xy+49y^2\)