<例題>次の計算をせよ。
(1) (a1/2+a−1/2)2
(2) (a1/2−b1/2)(a1/2+b1/2)
(3) (a1/3+b1/3)(a2/3+a1/3b1/3+a2/3)
(4) (a1/4−b1/4)(a1/4+b1/4)(a1/2+b1/2)(a+b)
(5) (log102)2+(log105)3+(log105)(log108)
<解答>(1) a1/2=x とおくと、a=x2
(a1/2+a−1/2)2=(x+x−1)2
=x2+2xx−1+x−2
=x2+2+x−2
=a+2+a−1
(2) a1/2=x, b1/2=y とおくと、a=x2, b=y2
(a1/2−b1/2)(a1/2+b1/2)=(x−y)(x+y)=x2−y2
=a−b
(3) a1/3=x,b1/3=y とおくと、a=x3, b=y3
(a1/3+b1/3)(a2/3−a1/3b1/3+a2/3)
=(x+y)(x2−xy+y2)
=x3+y3
=a+b
(4) a1/4=x,b1/4=y とおくと、
a1/2=x2, b1/2=y2,a=y4 ,b=y4
(a1/4−b1/4)(a1/4+b1/4)(a1/2+b1/2)(a+b)
=(x−y)(x+y)(x2+y2)(x4+y4)
=(x2−y2)(x2+y2)(x4+y4)
=(x4−y4)(x4+y4)
=x8−y8
=(a1/4)8−(b1/4)8)
=a2−b2
(5) log102=x、log105=y とおくと、
x+y=log102+log105=log1010=1
(log102)3+(log105)3+(log105)(log108)
=(log102)3+(log105)3+3(log105)(log102)
=x3+y3+3xy
=(x+y)(x2−xy+y2)+3xy
=(1)(x2−xy+y2)+3xy
=x2−xy+y2+3xy
=x2+2xy+y2
=(x+y)2
=(1)2
=1
|