[latex]b_n=b_1cdot q^{n-1}\frac{81}4cdotleft(-frac23ight)^{n-1}=4\left(-frac23ight)^{n-1}=frac{16}{81}\left(-frac23ight)^{n-1}=left(-frac23ight)^4\n-1=4\n=5\Pi POBEPKA:\b_5=b_1cdot q^4=frac{81}4cdotleft(-frac23ight)^4=frac{81}4cdotfrac{16}{81}=4[/latex]