S10=4S5
10a1+10x9d/2=4x(5a1+5x4d/2)
10a1+45d=20a1+40d
45d-40d=20a1-10a1
5d=10a1
d=2a1
a1/d=1/2
(2)
5分之1+√5分之1
=5/(1+√5)
=5(√5-1)/[(1+√5)(√5-1)]
=(5√5-5)/4 (4分之 5√5-5)
S10=4S5
10a1+10x9d/2=4x(5a1+5x4d/2)
10a1+45d=20a1+40d
45d-40d=20a1-10a1
5d=10a1
d=2a1
a1/d=1/2
(2)
5分之1+√5分之1
=5/(1+√5)
=5(√5-1)/[(1+√5)(√5-1)]
=(5√5-5)/4 (4分之 5√5-5)