When Robert was a small boy, he _______playing the piano.

Jim records his physical training every time so that he can         it and think about how he should improve it the next day. 

①（2x+y）²=4x²+y²，②（a﹣3b）²=a²﹣9b²，

③（﹣x﹣y）²=x²﹣2xy+y²，④（x﹣ $\frac{1}{2}$ ）²=x²﹣2x+ $\frac{1}{4}$ ．