Question 70
From the sum of x2−y2−1,y2−x2−1 and 1−x2−y2, subtract −(1+y2).
Sum of x2−y2−1,y2−x2−1 and 1−x2−y2
=x2−y2−1+y2−x2−1+1−x2−y2
On combining the like terms.
=x2−x2−x2−y2+y2−y2−1−1+1=−x2−y2−1
Now, subtract −(1+y2) from −x2−y2−1
=−x2−y2−1−[−(1+y2)]=−x2−y2−1+1+y2=−x2−y2+y2−1+1=−x2