\(214+\left[120-\left(214+120\right)\right]\)

\(=214+120-214-120\)

\(=0\)

\(\left(-321\right)-\left[\left(-321+35\right)-235\right]\)

\(=\left(-231\right)+231-35-235\)

\(=-270\)

thui khỏi nha , mik bt lm r

=6725600

a) 25 + ( - 13 + 8 )

= 25 - 13 + 8

= 20

b) 7 + ( - 12 + 43 ) - [ 2 + ( 19 - 34 ) ]

= 7 - 12 + 43 - [ 2+ 19 - 34 ]

= - 5 + 43 - 2 - 19 + 34

= 61

\(25+\left(-13+8\right)\)

\(=25-13+8\)

\(=12+8\)

\(=20\)

\(1+2+2^2+2^3+......+2^{90}\)

\(=\left(1+2+2^2+2^3\right)+....+\left(2^{87}+2^{88}+2^{89}+2^{90}\right)\)

\(=15+...+2^{87}\cdot15\)

\(=15\left(1+...+2^{87}\right)⋮15\)

1cc=1cm

1cc= 1000 cm.

Chúc bạn hk tốt!

Đặt \(10^k-1=19n\left(n\in Nsao\right)\)

\(\Rightarrow10^k=19n+1\Rightarrow\left(10^k\right)^3=\left(19n+1\right)^3\Rightarrow10^{3k}-1=\left(19n\right)^3+38n\)

Ta thấy\(\left(19n\right)^3⋮19;38n⋮19\Rightarrow\left(19n\right)^3+38n⋮19\)

Hay\(10^{3k}-1⋮19\)

\(10^{2k}-1=10^{2k}-10^k+10^k-1=\left(10^k-1\right)\left(10^k+1\right)⋮19\)

\(10^{3k}-1=10^{3k}-10^k+10^k-1=10^k\left(10^{2k}-1\right)+10^k-1⋮19\)

\(-1-2-3-...-99\)

\(=-\left(1+2+3+...+99\right)\)

\(=-\frac{\left(99+1\right).99}{2}=-\frac{100.99}{2}=-4950\)

( -1 ) + ( -2 ) + ( - 3 ) + ... + ( - 99 )

= - ( 1 + 2 + ... + 99 )

= - [ ( 99 + 1 ) . 99 : 2 ]

= - 4950

66 đường thẳng

số dt vẽ dc là:

[12.(12-1)]:2=66

Vậy...