Question

tính B=15a-17b+4a-2b.với a-b =20

Answer 1

Có : B=\(15a-17b+4a-2b\)

\(\Rightarrow B=15a+4a-17b-2b\)

\(\Rightarrow B=19a-19b\)

\(\Rightarrow B=19x\left(a-b\right)\)

Mà a-b=20

\(\Rightarrow B=19x20\)

\(\Rightarrow B=380\)

Answer 2

Thay b = a - 20

=> B = 15a - 17(a-20) - 4a - 2(a - 20)

        = 15a - 17a - 340 - 4a - 2a - 40

        = (15 - 17- 4 - 2)a - (340 - 40)

        = -8a - 300