a) tính tông nhớ đưa ra cách làm nha trả lời đúng vote 5 sao (6 mủ 2+ 7 mủ 2+ 8 mủ 2 +9 mủ 2 + 10 mủ 2 ) – ( 1 mủ 2 + 2 mủ 2 + 3 mủ 2+4 mủ 2+ 5 mủ 2 )

Đáp án:

Giải thích các bước giải:

 $(6^{2}+7^{2}+8^{2}+9^{2}+10^{2})-(1^{2}+2^{2}+3^{2}+4^{2}+5^{2})$

$=[6.(7-1)+7.(8-1)+8.(9-1)+9.(10-1)+10.(11-1)]-[1.(2-1)+2.(3-1)+3.(4-1)+4.(5-1)+5.(6-1)]$

$(6.7-6+7.8-7+8.9-8+9.10-9+10.11-10)-(1.2-1+2.3-2+3.4-3+4.5-4+5.6-5)$

$=(6.7+7.8+8.9+9.10+10.11)-(6+7+8+9+10)-(1.2+2.3+3.4+4.5+5.6)+(1+2+3+4+5)$

$=\dfrac{3.(6.7+7.8+8.9+9.10+10.11)}{3}-40-\dfrac{3.(1.2+2.3+3.4+4.5+5.6)}{3}+15$

$ $

$=\dfrac{3.6.7+7.8.3+8.9.3+9.10.3+10.11.3}{3}-\dfrac{1.2.3+2.3.3+3.4.3+4.5.3+5.6.3}{3}-25$

$ $

$=\dfrac{6.7.(8-5)+7.8.(9-6)+8.9.(10-7)+9.10.(11-8)+10.11.(12-9)}{3}-\dfrac{1.2.3+2.3.(4-1)+3.4.(5-2)+4.5.(6-3)+5.6.(7-4)}{3}-25$

$ $

$=\dfrac{6.7.8-5.6.7+7.8.9-6.7.8+8.9.10-7.8.9+9.10.11-8.9.10+10.11.12-9.10.11}{3}-\dfrac{1.2.3+2.3.4-1.2.3+3.4.5-2.3.4+4.5.6-3.4.5+5.6.7-4.5.6}{3}-25$

$ $

$=\dfrac{10.11.12-5.6.7}{3}-\dfrac{5.6.7}{3}-25$

$ $

$=370-70-25$

$=275$

$ $

$b) A=4+2^{2}+2^{3}+2^{4}+…+2^{20}$

$=2.(2+2+2^{2}+2^{3}+…+2^{19})$ $\vdots$ $2$