Question

[Tôi cần bạn]

Tìm x 

a) ( x - 25 ) : 15 = 20

b) 3 x x - 25 = 80   

Tính nhanh tổng sau

S= 1/2x3 + 1/3x4 + 1/4x5 + .... + 1/98x99 + 1/99x100

Bạn nào làm nhanh , mk sẽ tk cho

Answer 1

[Nguyễn Thanh Hiền]

a) \(\left(x-25\right):15=20\)

\(\Rightarrow x-25=20\times15\)

\(\Rightarrow x-25=300\)

\(\Rightarrow x=300+25\)

\(\Rightarrow x=325\)

Vậy x = 325

b) \(3\times x-25=80\)

\(\Rightarrow3\times x=80+25\)

\(\Rightarrow3\times x=105\)

\(\Rightarrow x=105:3\)

\(\Rightarrow x=35\)

Vậy x = 35

c) \(S=\frac{1}{2.3}+\frac{1}{3.4}+\frac{1}{4.5}+...+\frac{1}{98.99}+\frac{1}{99.100}\)

\(S=\frac{1}{2}-\frac{1}{3}+\frac{1}{3}-\frac{1}{4}+\frac{1}{4}-\frac{1}{5}+...+\frac{1}{98}-\frac{1}{99}+\frac{1}{99}-\frac{1}{100}\)

\(S=\frac{1}{2}-\frac{1}{100}\)

\(S=\frac{49}{100}\)

Vậy  \(S=\frac{49}{100}\)

_Chúc bạn học tốt_

Answer 2

[TAKASA]

a) (x-25):15=20

x-25=20×15

x-25=300

x=300+25

x=3325

b) 3×x-25=80

3×x=80+25

3×x=105

x=105:3

x=35

S=1/2×3 + 1/3×4 + 1/4×5 + ... + 1/98×99 + 1/99×100

S=1/2-1/3+1/3-1/4+1/4-1/5+...+1/98-1/99+1/99-1/100

S=1/2-1/100

S=50/100 - 1/100

S=49/100