Bài 1:
a) 3/1×2 + 3/2×3 + 3/3×4 +…+ 3/99×100
b) A = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90
c) 1995 x 1994 – 1/ 1993 x 1995 + 1994
Bài 1:
a) 3/1×2 + 3/2×3 + 3/3×4 +…+ 3/99×100
b) A = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90
c) 1995 x 1994 – 1/ 1993 x 1995 + 1994
Đáp án:
Giải thích các bước giải:
a) 3/1 x 2 + 3/2 x 3 + 3/3 x 4 +….+ 3/99×100
=3 x ( 1/1×2 + 1/2×3 + 1/3×4 + …+ 1/99×100)
=3 x (1/1-1/2+1/2-1/3+1/3-1/4+….+1/99-1/100)
= 3 x ( 1- 1/100)
= 3 x 99/100
= 297 / 100
b) A = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90
= 1 – 1/2 + 1- 1/6 + 1 – 1/ 12 + 1- 1/20 + 1 – 1/ 30 + 1 – 1/42 + 1- 1/56 + 1- 1/72 + 1-1/90
= ( 1 +1+1+1+1+1+1+1+1 ) – ( 1/1 – 1/ 2 + 1/3 – 1/2 +1/3 – 1/4 + 1/5 – 1/4 + 1/5 – 1/6 + 1/7 – 1/6 + 1/7 – 1/8 + 1/9 – 1/8 + 1/9 – 1/10)
= 9 – ( 1/1 – 1/10 )
= 9 – 9 /10
= 81 /10
c)1995 x 1994 – 1/ 1993 x 1995 + 1994
= 1995 x 1994 – 1/ ( 1994 -1 ) x 1995 + 1994
= 1995 x 1994 – 1/ 1994 x 1995 – 1995 + 1994
= 1995 x 1994 – 1/ 1994 x 1995 – 1
= 1
#Nhớ vote sao nhé
#Chúc em học tốt!
Bài 1:
a) 3/1×2 + 3/2×3 + 3/3×4 +…+ 3/99×100
= 3x(1/1×2+1/2×3+1/3×4+1/4×5+…+1/99×100)
= 3x(1/1-1/2+1/2-1/3+1/3-1/4+…+1/99-1/100)
= 3x(1-1/100)
= 3×99/100
= 297/100
b) A = 1/2 + 5/6 + 11/12 + 19/20 + 29/30 + 41/42 + 55/56 + 71/72 + 89/90
= 1-1/2+1-1/6+1-1/12+1-1/20+1-1/30+1-1/42+1-1/56+1-1/72+1-1/90
= 9-(1/2+1/6+1/12+1/20+1/30+1/42+1/46+1/72+1/90)
= 9-[1/(1×2)+1/(2×3)+1/(3×4)+1/(4×5)+1/(5×6)+1/(6×7)+1/(7×8)+1/(8×9)+
1/(9×10)]
= 9-(1-1/2+1/2-1/3+1/3-1/4+1/4-1/5+1/5-1/6+1/6-1/7+1/7-1/8+1/8- 1/9+1/9-1/10)
= 9-(1-1/10)
= 9-9/10
= 81/10
c) 1995 x 1994 – 1/ 1993 x 1995 + 1994
= 1995 x 1994 – 1/(1994 – 1) x 1995 + 1994
= 1995 x 1994 – 1/1994 x 1995 – 1995 + 1994
= 995 x 1994 – 1/1994 x 1995 – 1
= 1
( Cho mik câu tlhn nha )