(X+5)^4+(x-4)^4=(2x+1)^4
(=)(X+5)^4+(x-4)^4-(2x+1)^4=0
(=)[(x+5)²+(x-4)²]²-[(2x+1)²]²=0
(=)(x²+10x+25+x²-8x+16)²-(4x²+4x+1)²=0
(=)(2x²+2x+41)²-(4x²+4x+1)²=0
(=)(2x²+2x+41-4x²-4x-1)(2x²+2x+41+4x²+4x+1)=0
(=)(-2x²-2x+40)(6x²+6x+42)=0
(=)-2(x²+x-20)(6x²+6x+42)=0
(=)-2(x²+5x-4x-20)(6x²+6x+42)=0
(=)-2[x(x+5)-4(x+5)](6x²+6x+42)=0
(=)-2(x+5)(x-4)(6x²+6x+42)=0
x+5=0=)x=-5
x-4=0=)x=4
6x²+6x+42=0=)vô lý
