用含有字母的式子表示下面的数量关系.①30与2个a的和______②两个b的积的一半______③比x的5倍少9的数______④x与12的差的5倍______⑤m除15与n的和______⑥a和b的和乘它的差______.
(1)30+2×a=30+2a, (2)b×b÷2, =
(3)x×5-9, =5x-9, (4)(x-12)×5, =5x-12×5, =5x-60, (5)(15+n)÷m, (6)(a+b)×(a-b), =a×a-a×b+a×b-b×b, =a 2 -b 2 , 故答案为:30+2a,
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