Answer:
9.271 inches.
Explanation:
Let AC be the length of original line segment and point B divides it into two segments such that AB is longer and BC is smaller segment.
![AB+BC=AC=15](https://img.qammunity.org/2020/formulas/mathematics/high-school/r6uao1uyfh8sc2apayzv8bhflf9gyc9jxw.png)
We can describe the golden ratio as:
![(AB)/(BC)=(AC)/(AB)=1.618](https://img.qammunity.org/2020/formulas/mathematics/high-school/eixn0z9lmspnyo8p8ppjswxiux9ip2q0fh.png)
![(AC)/(AB)=1.618](https://img.qammunity.org/2020/formulas/mathematics/high-school/yh89oizjjncxu25eh0gwjcoopbnv9mobxi.png)
![(15)/(AB)=1.618](https://img.qammunity.org/2020/formulas/mathematics/high-school/dkbiktoufmr3xgl12mbo0nwyo61aca8aq1.png)
![(15)/(1.618)=AB](https://img.qammunity.org/2020/formulas/mathematics/high-school/u9m7w9uf2o0ifyba8m1i7n1j6d2zm5ci1s.png)
![9.270704=AB](https://img.qammunity.org/2020/formulas/mathematics/high-school/dypwzhfr0klw059nesrul7hxw90vk2mlwa.png)
![AB=9.271](https://img.qammunity.org/2020/formulas/mathematics/high-school/h2ug63hv8c2eiqsno44t9wf8j2wmjidzg4.png)
We can verify our answer as:
![AB+BC=15](https://img.qammunity.org/2020/formulas/mathematics/high-school/9k47zuid1b84i6t4wmc4m89v40qrsqljni.png)
![9.271+BC=15](https://img.qammunity.org/2020/formulas/mathematics/high-school/mo8b0pt8mfs1vo4a09zxf431bz7wt28iux.png)
![9.271-9.271+BC=15-9.271](https://img.qammunity.org/2020/formulas/mathematics/high-school/3afq7d2uxscue42ye0j9zwc8hnresb0913.png)
![BC=5.729](https://img.qammunity.org/2020/formulas/mathematics/high-school/ci43wr2rkobrbp8dbb613tmwzlz84st6nn.png)
![(AB)/(BC)=1.618](https://img.qammunity.org/2020/formulas/mathematics/high-school/snp2b1rh1l0w0pu54fsmhcsm3x2psfdhmg.png)
![(9.271)/(5.729)=1.618](https://img.qammunity.org/2020/formulas/mathematics/high-school/1xubwmdbptryh1m0sstgds1sfypyry8uj3.png)
![1.618=1.618](https://img.qammunity.org/2020/formulas/mathematics/high-school/mcto10yli5z1mklxhhpfjk7cp4jnb6xhpd.png)
Hence proved.
Therefore, the length of the longer side would be 9.271 inches.