7A= 45 hs

7B=35 hs

7C=40 hs

13,=\(\dfrac{47}{21}\)

14, ⇒x=\(\dfrac{4}{5}\)-\(\dfrac{3}{4}\)

⇒x=\(\dfrac{16}{20}\)-\(\dfrac{15}{20}\)

⇒x=\(\dfrac{1}{20}\)

841

=9

= 1(2-1)+2(3-1)+3(4-1)+....+100(101-1)

=1.2-1.1+2.3-2.1+3.4-3.1+....+100.101-100

=(1.2+2.3+3.4+...+100.101)-(1+2+3+...+100)

=3(1.2+2.3+3.4+...+100.101)/3 -(1+2+3+..+100)

=[1.2.3+2.3.(4-1)+3.4.(5-2)+...+100.101(102-99)]/3 -(1+2+3+..+100)

=(1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+...+100.101.102-99.100.101)/3-5050

=\(\dfrac{100.101.102}{3}\)-5050=343400-5050=338350

mik làm vậy nếu sai đừng trách mik 

=\(\dfrac{3}{32}\)