$3)u_{10} =u_1+9d\\ =>d=\dfrac{u_{10}-u_1}{9}=2\\ =>C\\ 4)(u_n)=\dfrac{1}{2};\dfrac{1}{2^2};\dfrac{1}{2^3} \ldots\\ =>C\\ 5)A:u_1=1;q=(-2)^{n-1}\\ 6)u_3=u_1q^2;u_5=u_1q^4\\ =>\dfrac{u_5}{u_3}=q^2=9=>q=3\\ =>C\\ 7)(u_n)=13;2x;23\\ \dfrac{23}{13}=q^2=>q=\dfrac{23}{13}\\ =>2x=13.\dfrac{23}{13}=>x=\dfrac{\sqrt{299}}{2}$