Bài 1: Tính nhanh
A = (a x 7 + a x 8 – a x 15) : (1 + 2 + 3 + …….. + 10)
B = (18 – 9 x 2) x (2 + 4 + 6 + 8 + 10)
Bài 2: Tìm x
X x 5 + 122 + 236 = 633
(X : 12) x 7 + 8 = 36
Bài 1: Tính nhanh A = (a x 7 + a x 8 – a x 15) : (1 + 2 + 3 + …….. + 10) B = (18 – 9 x 2) x (2 + 4 + 6 + 8 + 10) Bài 2: Tìm x X x 5 + 122 + 236 =
By Maya
1 ,
A=(a x 7 + a x 8 – a x 15) : (1+2+3+…+10)
= [a x ( 7+ 8 – 15)] : [ (1+9) + ( 2 + 8 ) + (3 + 7) + (4+6)+(5+10)
= [a x ( 15 – 15 )] : ( 10 + 10 +10+10+15)
= (a x 0 ) : 55
= 0 : 55
= 0
B= (18 – 9 x 2) x (2 + 4 + 6 + 8 + 10)
= (18-18) x [(2+6) + (4+8) + 10]
= 0 x (8 + 12 + 10)
= 0 x (20 + 10)
= 0 x 30
= 0
2 ,
a) X x 5 + 122 + 236 = 633
X x 5 + 122 = 633 – 236
X x 5 + 122 = 397
X x 5 = 397 – 122 = 275
X = 275 : 5
X = 55
b) (X : 12) x 7 + 8 = 36
(X : 12) x 7 = 36 – 8 = 28
(X : 12) = 28 : 7
X : 12 = 4
X = 4 x 12
X = 48
A = (a x 7 + a x 8 – a x 15) : (1 + 2 + 3 + …….. + 10)
= [ a×(7 +8 -15)] ÷ (1 + 2 + 3 + …….. + 10)
= (a×0) ÷ (1 + 2 + 3 + …….. + 10)
= 0 ÷ (1 + 2 + 3 + …….. + 10)
= 0
B = (18 – 9 x 2) x (2 + 4 + 6 + 8 + 10)
= (18 -18) × (2 + 4 + 6 + 8 + 10)
= 0 × (2 + 4 + 6 + 8 + 10)
= 0
X × 5 + 122 + 236 = 633
⇒ X × 5 + 358 = 633
⇒ X × 5 = 633 -358
⇒ X × 5 = 275
⇒ X = 275 ÷ 5
⇒ X = 55
Vậy X = 55
(X : 12) × 7 + 8 = 36
⇒ (X÷ 12) ×7 = 36 -8
⇒ (X÷ 12) ×7 = 28
⇒ (X÷ 12) = 28 ÷ 7
⇒ X÷ 12 = 4
⇒ X = 4 × 12
⇒ X = 48
Vậy X = 48