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a. x-115
b.x+388
Tick mình đi!!!!!!!!!!!! Chúc bạn năm mới vui vẻ
a, x+45-[90+(-20)+5-(-45)]
=x+45-[90+(-20)+5+45]
=x+45-90+20-5-45
=x+(45-45)-(90+20)-5
=x+0-110-5
=x-115
vay x-115
b, x+(294+13)+(94-13)
=x+307+81
=x+388
vay x+388
\(A=1+4+4^2+...+4^{90}\)
\(4A=4+4^2+4^3+...+4^{90}+4^{91}\)
\(4A-A=\left(4+4^2+4^3+...+4^{91}\right)-\left(1+4+4^2+...+4^{90}\right)\)
\(3A=4+4^2+4^3+...4^{91}-1-4-4^2-...-4^{90}\)
\(3A=4^{91}-1\)
\(A=\frac{4^{91}-1}{3}\)
t i c k nha ^^
4A= 4+42+43+....+491
4a-4=(4+42+43+...+491)-(1+4+42+...+490)
3a=491-1
a=(491-1)/3
ghi cả cách giải nữa chứ,ghi mỗi đáp án thì làm sao hiểu được
a) x+45-[90+(-20)+5-(-45)]
=x+45-120
=x+-75
b) x+(294+13)+(94-13)
=x+307+81
=x+388
a) x+45-[90+(-20)+5-(-45)]
=x+45-[(90-20)+(5+45)]
=x+45-[70+50]
=x+45-120
=x+(45-120)
=x-75
b) x+(294+13)+(94-13)
=x+307+81
=x+(307+81)
=x+388
A = 1 + \(\dfrac{1}{3^2}\) + \(\dfrac{1}{3^3}\) +.......+\(\dfrac{1}{3^{n-1}}\) + \(\dfrac{1}{3^n}\)
3\(\times\) A = 3 + \(\dfrac{1}{3}\) + \(\dfrac{1}{3^2}\) + \(\dfrac{1}{3^3}\)+........+ \(\dfrac{1}{3^{n-1}}\)
3A - A = 3 + \(\dfrac{1}{3}\) - 1 - \(\dfrac{1}{3^n}\)
2A = \(\dfrac{7}{3}\) - \(\dfrac{1}{3^n}\)
A = ( \(\dfrac{7}{3}\) - \(\dfrac{1}{3^n}\)): 2
A = \(\dfrac{7.3^{n-1}-1}{3^n}\) : 2
A = \(\dfrac{7.3^{n-1}-1}{2.3^n}\)
B = \(\dfrac{1}{2}\) - \(\dfrac{1}{2^2}\) + \(\dfrac{1}{2^3}\) - \(\dfrac{1}{2^4}\)+......+\(\dfrac{1}{2^{99}}\) - \(\dfrac{1}{2^{100}}\)
2B = 2 - \(\dfrac{1}{2}\) + \(\dfrac{1}{2^2}\) - \(\dfrac{1}{2^3}\)+ \(\dfrac{1}{2^4}\)-.......-\(\dfrac{1}{2^{99}}\)
2B + B = 2 - \(\dfrac{1}{2^{100}}\)
3B = 2 - \(\dfrac{1}{2^{100}}\)
B = ( 2 - \(\dfrac{1}{2^{100}}\)): 3
B = \(\dfrac{2.2^{100}-1}{2^{100}}\) : 3
B = \(\dfrac{2^{101}-1}{3.2^{100}}\)
a, a+8
b, b+45