$\perp \perp $ a,b\in R;n\in N^{\ast }$$ \frac{a^n+b^n}{4}\ge {\left(\frac{a+b}{4}\right)}^n$$ \frac{}{}$$ P\left(x\right)=x^2+mx-9$$ x=1$$ P\left(x\right)$$ m=0$$ P\left(x\right)$$ m=0$$ P\left(x\right)$$ \frac{x.\left|x-2\right|}{x-2}$$ \ne $$ \widehat{AOB}$$ {}^{}$$ \perp $$ \perp $$ \Delta $$ \Delta $$ \Delta $$ \Delta $$ \Delta $$ \hat{A}={90}^0$$ \hat{B}={45}^0$$ ABC$$ \hat{B}=\hat{C}$$ A$$ BC$$ D.$$ DB=DC,AB=AC.$$ $$ ABC$$ \hat{A}=\hat{B}={60}^{\circ }$$ C.$$ \mathrm{AB}//\mathrm{Cx}$$ k$$ \frac{1}{{\sin }^2\frac{\pi }{4k+2}}+\frac{1}{{\sin }^2\frac{3\pi }{4k+2}}+\frac{1}{{\sin }^2\frac{5\pi }{4k+2}}+\cdots +\frac{1}{{\sin }^2\frac{(2k-1)\pi }{4k+2}}=2k(k+1$$ \frac{1}{4}:6$$ \frac{5}{2}:3$$ \frac{a}{b}=\frac{c}{d}.$$ \frac{5}{6}=\frac{25}{30}$$ \frac{ka}{kb}=\frac{a}{b}$$ k\in Q;k\ne 0$$