[tex]\displaystyle\bf\\a_{1} =4\\\\a_{20} =42\\\\\\S_{20} =\frac{a_{1} +a_{20} }{2} \cdot 20=(a_{1} +a_{20} )\cdot 10=(4+42)\cdot 10=46\cdot 10=460\\\\\\Otvet: \ S_{20} =460[/tex]