A.) Check whether the following signals are periodic and determine the fundamental period. i.) x1(t)=cos3(t) ii.) x2(t)=sin(t)+cos(t) iii.) x3[n]=k=([nk]+[n2k]) iv.) x4[n]=cos(0.7n) B.) Use clear and detailed mathematical steps to check the linearity, causality, stability and time-inv of the following system y(t)=x(t)x(t+5)+3x(t2)+12