Bài 6:
Ta có: $\frac{2.3+3.4+4.5+5.8+6.7+7.2+8.9+2a}{40}=5,65$
⇒$\frac{206+2a}{40}=5,65$ =>$5,65.40=206+2a$
⇒$206+2x=226$
⇒$2a=20$
⇒$a=10$
Bài 7:
Ta có: $\frac{5.2+6.5+9n+10}{2+5+n+1}=6,8$
⇒$\frac{50+9n}{8+n}=6,8$
⇒$6,8.(8+n)=50+9n$
⇒$54,4+6,8n=50+9n$
⇒$6,8n-9n=50-54,4$
⇒$-2,2n=-4,4$
⇒$n=2$