x + 78 + x = 198

X x 2 + 78 = 198

X x 2         = 198 - 78

X x 2         = 120

             x  = 120 : 2

             x =  60

sk mình mình làm cho

30x2+30x5+30x3

30x(5+3+2)

30x 10

300

\(S=\frac{1}{1.2}+\frac{1}{2.3}+\frac{1}{3.4}+...+\frac{1}{99.100}\)

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

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

\(S=\frac{99}{100}\)

a) ( 426 + x ) : 5 = 120

426 + x = 120 x 5

426 + x = 600

x = 600 - 426

x = 174

b) 5844 : x - 159 = 328

5844 : x = 328 + 159

5844 : x = 487

x = 5844 : 487

x = 12

c) 3²x + 1 = 27

3²x = 27 - 1

3²x = 26

9x = 26

x = 26 : 9

x = \(\frac{26}{9}\)

d) ( 3x - 1 )² = 64

3x - 1 = 8

3x = 8 + 1

3x = 9

x = 9 : 3 

x = 3