Question

[Linh Chi]

Bài 2 : Tìm x thuộc N . biết :

a ) ( 4x + 5 ) : 3 - 121 : 11 = 4

b ) 1 + 3 + 5 + ... + x = 1600

Answer 1

[Ngoc Han ♪]

a ) \(\left(4x+5\right)\div3-121\div11=4\)

\(\left(4x+5\right)\div3-11=4\)

\(\left(4x+5\right)\div3=4+11\)

\(\left(4x+5\right)\div3=15\)

\(\left(4x+5\right)=15\cdot3\)

\(4x+5=45\)

\(4x=45-5\)

\(4x=40\)

\(x=10\)

Answer 2

[Xyz OLM]

(4x + 5) : 3 - 121 : 11 = 4

=> (4x + 5) : 3 - 11 = 4

=> (4x + 5) : 3 = 15

=> 4x + 5 = 45

=> 4x = 40

=> x = 10

b) 1 + 3 + 5 + ... + x = 1600

=>[(x - 1) : 2 + 1] . (x + 1) : 2 = 1600

=> \(\left(\frac{x}{2}-\frac{1}{2}+1\right).\frac{x+1}{2}=1600\)

=> \(\frac{x+1}{2}.\frac{x+1}{2}=1600\)

=> \(\left(\frac{x+1}{2}\right)^2=1600\)

=> \(\frac{x+1}{2}=40\)

=> x + 1 = 80

=> x = 79