Tính giá trị của biểu thức: (20^2 + 18^2+ 16^2...+4^2+2^2)-(19^2+17^2+15^2+...+3^2+1^2)
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N = (202 - 192) + (182 - 172) + ...+ (42 - 32) + (22 - 12)
= (20 - 19).(20 + 19) + (18 - 17)(18 + 17) +...+ (4 -3).(4 +3) + (2-1)(2+1)
= 39 + 35 + ...+ 7 + 3
Số số hạng: (39 - 3): 4 + 1 = 10
=> N = (39 + 3).10 : 2 = 210
+) Ở đây: sd công thức: (a-b).(a+b) = a2 - b2
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Gọi A là biểu thức trên
A=2x(1/1x2+1/2x3+....+1/19.20)
A=2x(1-1/2+1/2-1/3+1/3-1/4+....+1/19-1/20)
A=2x(1-1/20)
A=2x19/20
A=19/10
Vậy giá trị biểu thức trên là 19/10
Chúc em học tốt^^
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\(\dfrac{8^2.4^5}{2^{20}}\) = \(\dfrac{\left(2^3\right)^2\left(2^2\right)^5}{2^{20}}\) = \(\dfrac{2^6.2^{10}}{2^{20}}\) = \(\dfrac{2^{16}}{2^{20}}\) = \(\dfrac{1}{2^4}\) = \(\dfrac{1}{16}\)
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\(B=\frac{7.6^{10}.2^{20}.3^6-2^{19}.6^{15}}{9.6^{19}.2^9-4.3^{17}.2^{26}}\)
\(B=\frac{7.\left(2.3\right)^{10}.2^{20}.3^6-2^{19}.\left(2.3\right)^{15}}{3^2.\left(2.3\right)^{19}.2^9-2^2.3^{17}.2^{26}}\)
\(B=\frac{7.2^{30}.3^{16}-2^{34}.3^{15}}{3^{21}.2^{11}-2^{28}.3^{17}}\)
\(B=\frac{2^{30}.3^{15}.\left(7.3-2^4\right)}{3^{17}.2^{11}.\left(3^4-2^{17}\right)}\)
\(B=\frac{2^{19}.\left(21-2^4\right)}{3^2.\left(3^4-2^{17}\right)}\)
@_@ tự tính!
\(B=\frac{7.\left(2.3\right)^{10}.2^{20}.3^6-2^{19}.\left(2.3\right)^{15}}{3^2.\left(2.3\right)^{19}.2^9-4.3^{17}.2^{26}}\\ B=\frac{7.2^{10}.3^{10}.2^{20}.3^6-2^{19}.2^{15}.3^{15}}{3^2.2^{19}.3^{19}.2^9-2^2.3^{17}.2^{26}}\\ B=\frac{7.2^{30}.3^{16}-2^{34}.3^{15}}{3^{21}.2^{28}-2^{28}.3^{17}}\)
\(B=\frac{7.2^{30}.3.3^{15}-2^{34}.3^{15}}{3^{15}.3^6.2^{28}-2^{28}.3^{15}.3^2}\\ B=\frac{3^{15}.\left(7.2^{30}.3-3^{34}\right)}{3^{15}.\cdot\left(3^6-3^2\right)}\\ B=\frac{7.2^{30}.3-3^{34}}{3^6-3^2}\)
\(\left(20^2+18^2+16^2+......+4^2+2^2\right)-\left(19^2+17^2+.....+3^2+1^2\right)\)
\(=20^2-19^2+18^2-17^2+......+2^2-1^2\)
\(=\left(20-19\right)\left(20+19\right)+\left(18-17\right)\left(18+17\right)+.......+\left(2-1\right)\left(2+1\right)\)
\(=39+35+....+7+3\)
\(=\left(39+3\right)\left[\left(39-3\right):4+1\right]:2=210\)