Expand {(a+b)4+(a−b)4} and use it to ealuate
(x2+√1−x2)4+(x2−√1−x2)4
We have
(a+b)4+(a−b)4=[4C0a4+4C1a3b+4C2a2b2+6C3ab3+4C4b4]+[4C0a4−4C1a3b+4C2a2b2−4c3ab3+4C4b4]=2[4C0a4+4C2a2b2+4C4b4]Putting√1−x2=y,we get(x2+√1−x2)4+(x2−√1−x2)4(x2+y)4+(x2−y)4=2[4C0(x2)4+4C2(x2)2y2+4C4y4]=2[x6+6x4y2+y4]=2[x8+6x4(1−x2)+(1−x2)2]=2[x8+(6x4−6x6)+(1−2x2+x2)]=2[x8−6x6+7x4−2x2+1]=2x8−12x6+14x4−4x2+2