Why does d = a₁R + a₂R + ... + aₙR imply each aᵢ is in (d)?

[Math Amateur]

I am reading Paul E. Bland's book, "Rings and Their Modules".

I am focused on Section 4.3: Modules Over Principal Ideal Domains ... and I need some help to fully understand the proof of part of Proposition 4.3.3 ... ...

Proposition 4.3.3 reads as follows:View attachment 8247
View attachment 8248
In the above proof by Bland we read the following:

"... ... If $$(d) = a_1 R + a_2 R + \ ... \ ... \ + a_n R$$, then each $$a_i$$ is in $$(d)$$ ... ... "Can someone please explain how $$(d) = a_1 R + a_2 R + \ ... \ ... \ + a_n R$$ implies each $$a_i$$ is in $$(d)$$ ... ..Peter

 

[Euge]

Hi Peter,
$$a_1 = a_1\cdot 1 + a_2 \cdot 0 + \cdots + a_n \cdot 0 \in (d)$$ and similarly for the other $a_i$.