حل مسالة 3 ص 120 رياضيات 4 متوسط؟

x+y=20…….1                                                                                                                                       x²-y²= 40………2                                                                                                                       
 y=20-x……..3                        :من المعادلة 1نجد                                                                             x²-(20-x)²=40                                                                                                                                     x²-(400+x²-40x)=40                                                                                                            x²-400-x²+40x=40                                                                                                                           40x=440                       x=440/40
x=11
y=20-x    y=20-11
y=9