Tìm a biết: ( a + 2 ) + ( a + 4 ) + ( a + 5 ) + ( a + 6 ) + ( a + 8 )

Tìm a biết:

( a + 2 ) + ( a + 4 ) + ( a + 5 ) + ( a + 6 ) + ( a + 8 ) = 90.

• A. a = 13

Ta có :

( a + 2 ) + ( a + 4 ) + ( a + 5 ) + ( a + 6 ) + ( a + 8 )

= ( a + a + a + a + a ) + ( 2 + 4 + 5 + 6 + 8 )

= 5 x a + 25

Hay 5 x a + 25 = 90 

5 x a = 90 - 25 = 65

Suy ra a = 65 : 5 = 13.

Vậy a = 13