4 2-2 +(3-7)-2
3/4*6/7除以(1/2+2)-2/7
2-2^2-2^3-2^4-2^5-2^6-2^7-2^8-2^9.-2^20
2-2^2-2^3-2^4-2^5-2^6-2^7-2^8-2^9+2^10
2-2^2-2^3-2^4-2^5-2^6-2^7-2^8-2^9.-2^20=?
2-2^2-2^3-2^4-2^5-2^6-2^7-2^8-2^9.+2^20 过程
计算 3/7-4/9+2/7
求1×2+4×2^2+7×2^3+.+(3n-2)×2^n
求和sn=1*2+4*2^2+7*2^3+...+(3n-2)*2^n
2/1*4+2/4*7+2/7*10+2/10*13+.+2/(3n-2)(3n+1)
合并同类项:3x^2-[7x-2(4x+2)+2x^2]-x^2
1^2+4^2+6^2+7^2=2^2+3^2+5^2+8^2 找规律
2^13+2^12+2^11+2^10+2^9+2^8+2^7+2^6+2^5+2^4+2^3+2^2+2+1