<=> (a+15)(1/1.2 +...+1/99.100)=297/10
<=>(a+15)(1/1-1/2+...=1/99-1/100)=297/100
<=> (a+15)99/100=297/100
<=>a+15=3
<=>a=-12
Tách a+15 ra ngoài thanh một thừa số
còn lại tự giải nốt
kết quả ra 15
\(\frac{a+15}{1.2}+\frac{a+15}{2.3}+...+\frac{a+15}{99.100}=\frac{297}{10}\)
\(\Rightarrow\)\(\left(a+15\right)\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{99.100}=\frac{297}{10}\)
\(\Rightarrow\left(a+15\right)\left(1-\frac{1}{2}+...+\frac{1}{99}-\frac{1}{100}\right)=\frac{297}{10}\)
\(\Rightarrow\)\(\left(a+15\right)\left(1-\frac{1}{100}\right)=\frac{297}{10}\)
\(\Rightarrow\)(a+15).\(\frac{99}{100}\)=\(\frac{297}{10}\)
\(\Rightarrow\)a+15=0.003
\(\Rightarrow\)a=-14.997
\(\Rightarrow\left(a+15\right)\left(\frac{1}{1.2}+\frac{1}{2.3}+...+\frac{1}{99.100}\right)=\frac{297}{10}\)
\(\Rightarrow\left(a+15\right)\left(1-\frac{1}{2}+...+\frac{1}{99}-\frac{1}{100}\right)=\frac{297}{10}\)
\(\Leftrightarrow\)\(\left(a+15\right)\left(1-\frac{1}{100}\right)=\frac{297}{10}\)
\(\Rightarrow\left(a+15\right).\frac{99}{100}=\frac{297}{10}\)
\(\Leftrightarrow a+15=30\)
\(\Leftrightarrow a=15\)