Question

giúp bài 5 ạ

Answer 1

a: \(A=15-16+17-18+19-20+21-22+23-24\)

=(15-16)+(17-18)+(19-20)+(21-22)+(23-24)

=(-1)+(-1)+(-1)+(-1)+(-1)

=-5

b: B=10-11+12-13+...+98-99+100

=(10-11)+(12-13)+...+(98-99)+100

=(-1)+(-1)+...+(-1)+100

Từ 10 đến 99 có (99-10):1+1=99-10+1=90(số)

=>\(B=\left(-1\right)\cdot\dfrac{90}{2}+100=-45+100=55\)

c: \(C=10+11-12-13+14+15-16-17+18+19-20-21\)

=(10-12)+(11-13)+(14-16)+(15-17)+(18-20)+(19-21)

=(-2)+(-2)+...+(-2)

=-12

d: \(D=1-2-3+4+5-6-7+8+9-10-11+12+13-14-15+16\)

=(1-2-3+4)+(5-6-7+8)+(9-10-11+12)+(13-14-15+16)

=0+0+0+0

=0

f: \(E=20-21-22+23+24-25-26+27+...+60-61-62+63\)

\(=\left(20-21-22+23\right)+\left(24-25-26+27\right)+...+\left(60-61-62+63\right)\)

\(=0+0+...+0\)

=0

g: Sửa đề: \(G=1+3-5-7+9+11-13-15+...-397-399\)

\(=\left(1+3-5-7\right)+\left(9+11-13-15\right)+...+\left(393+395-397-399\right)\)

\(=\left(-8\right)+\left(-8\right)+...+\left(-8\right)\)

Dãy số này có \(\dfrac{399-1}{2}+1=\dfrac{398}{2}+1=199+1=200\left(số\right)\)

=>\(G=\left(-8\right)\cdot\dfrac{200}{4}=-8\cdot50=-400\)