Đáp án:
`a)224x²-180x-125`
`b)6a²+3b²+2c²`
`c)5x+81`
Giải thích các bước giải:
`a)(4x-5)(25+40x+16x)`
`=(4x-5)(56x+25)`
`=4x(56x+25)-5(56x+25)`
`=224x²+100x-280x-125`
`=224x²+(100x-280x)-125`
`=224x²-180x-125`
`b)(a+b+c)²+(a+b-c)²+(2a-b)²`
`=[(a+b)+c]²+[(a+b)-c]²+(2a)²-2.2a.b+b²`
`=(a+b)²+2.(a+b).c+c²+(a+b)²-2.(a+b).c+c²+4a²-4ab+b²`
`=a²+2ab+b²+2ac+2bc+c²+a²+2ab+b²-2ac-2bc+c²+4a²-4ab+b²`
`=(a²+a²+4a²)+(2ab+2ab-4ab)+(b²+b²+b²)+(2ac-2ac)+(2bc-2bc)+(c²+c²)`
`=6a²+3b²+2c²`
`c)(2x+9)²-x(4x+31)`
`=(2x)²+2.2x.9+9²-4x²-31x`
`=4x²+36x+81-4x²-31x`
`=(4x²-4x²)+(36x-31x)+81`
`=5x+81`