Đáp án:
1. Rút gọn
a)(x+5)²-(3x)²=(x+5-3x)(x+5+3x)=(5-2x)(4x+5)=20x-8x²+25-10x=-8x²-10x+25
b)(4-x)²-(x-3)²=(4-x-x+3)(4-x+x-3)=(7-2x).1=7-2x
c)(x-5)(x+5)-(x+5)²=(x+5)(x-5-x-5)=(x+5)(-10)=-10x-50
d)(x-3)²-(x-4)(x+4)=x²-6x+9-(x²-16)=x²-x²-6x+9+16=25-6x
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