【单选题】Which of the following statements is correct ? (1) There are two types of stress, normal stress( σ ) and shear stress( τ ). (2) On one section, the normal stress( σ ) and the shear stress( τ ) must be...
【简答题】When David is twice as old as he is now he will be four times as old as his daughter Jane will be in five years time. If in 1990, four years ago, he was four times as old as his daughter, in which yea...