\(2021S=2021+2021^2+2021^3+...+2021^{40}\\ 2021S-S=2021+2021^2+2021^3+...+2021^{40}-1-2021-2021^2-...-2021^{39}\\ 2020S=2021^{40}-1\\ S=\dfrac{2021^{40}-1}{2020}\)

\(4\cdot5^2-3^2\cdot\left(2021^0+3^2\right)\)
\(=4\cdot25-9\cdot\left(1+9\right)\)
\(=100-9\cdot10\)
\(=100-90\)
\(=10\)

4. 5²- 3². ( 2021⁰+ 3²)

= 4 . 25 - 9 . ( 1 + 9 )

= 100 - 9 . 10

= 100-90

= 10

\(\text{a)A=2021+{750-[2021-(+50)}\)

\(A=2021+750-2021+50\)

\(A=\left(2021-2021\right)+\left(750+50\right)\)

\(A=0+800\)

\(A=800\)

\(\text{b)B=-215.[19+(-1236)]+215.(19-236)}\)

\(B=-215.19+215.1236+215.19-215.236\)

\(B=19.\left(-215+215\right)+215.\left(1236-236\right)\)

\(B=19.0+215.1000\)

\(B=0+215000\)

\(B=215000\)

học tốt

\(a,-215-358+215+\left(-42\right)\\ =\left(-215+215\right)+\left(-358-42\right)\\ =0-400\\ =-400\\ b,13\cdot\left(23+22\right)-3\cdot\left(17+28\right)\\ =13\cdot45-3\cdot45\\ =45\cdot\left(13-3\right)\\ =45\cdot10\\ =450\)

\(=95-95:\left\{6^2-\left(4^2+1\right)\right\}\)

\(=95-5=90\)

\(A>B\)

giúp vơi

khó difficult