a: \(\left\{{}\begin{matrix}u5-u1=15\\u4-u1=6\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}u1\cdot q^4-u1=15\\u1\cdot q^3-u1=6\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}u1\left(q^4-1\right)=15\\u1\left(q^3-1\right)=6\end{matrix}\right.\Leftrightarrow\dfrac{q^4-1}{q^3-1}=\dfrac{5}{2}\)
=>\(2\left(q^4-1\right)=5\left(q^3-1\right)\)
=>\(2q^4-2-5q^3+5=0\)
=>\(2q^4-5q^3+3=0\)
=>\(2q^4-2q^3-3q^3+3=0\)
=>\(2q^3\left(q-1\right)-3\left(q-1\right)\left(q^2+q+1\right)=0\)
=>\(\left(q-1\right)\left(2q^3-3q^2-3q-3\right)=0\)
=>\(\left[{}\begin{matrix}q=1\\q\simeq2,39\end{matrix}\right.\)
=>\(u1=\dfrac{6}{q^3-1}\simeq\dfrac{6}{2.39^3-1}\simeq0,47\)
b: \(\left\{{}\begin{matrix}u1-u3+u5=65\\u1+u7=325\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}u1-u1\cdot q^2+u1\cdot q^4=65\\u1+u1\cdot q^6=325\end{matrix}\right.\)
=>\(\left\{{}\begin{matrix}u1\cdot\left(1-q^2+q^4\right)=65\\u1\left(1+q^6\right)=325\end{matrix}\right.\)
=>\(\dfrac{1-q^2+q^4}{1+q^6}=\dfrac{65}{325}=\dfrac{1}{5}\)
=>\(\dfrac{1}{q^2+1}=\dfrac{1}{5}\)
=>\(q^2+1=5\)
=>q^2=4
=>q=2 hoặc q=-2
TH1: q=2
=>\(u1=\dfrac{325}{q^6+1}=5\)
TH2: q=-2
=>\(u1=\dfrac{325}{\left(-2\right)^6+1}=5\)
