已知4x=3y,求代数式(x﹣2y)2﹣(x﹣y)(x+y)﹣2y2的值.
— Can you play the piano?
—Yes, I . I often practice it on weekends.
电路图如图所示,电源电动势为20V,内阻r=3Ω,滑动变阻器的最大阻值为20Ω,定值电阻R0=2Ω.则下列说法正确的是( )