I'm Chinese, so I don't know if I can explain it clearly.
2. ∵f(a)=2/a, f(b)=2/b,
∴f(a)+f(b)=2/a+2/b=2(a+b)/ab;
but f(a+b)=2/(a+b);
if a=2,b=4, then 2(a+b)/ab=3/2; 2/(a+b)=1/3; so D is wrong.
3. If we know both of them, then ∠RQS=∠RSQ, ∠TSU=∠TUS.
∵∠P=90 ∴ ∠R+∠T=90
and∵∠R=180-2∠RSQ ∠T=180-2∠TSU.
∴∠R+∠T=(180-2∠RSQ)+(180-2∠TSU)=90
∴ ∠RSQ+∠TSU=135, so ∠X=180-135=45.
Hope u can understand these~~
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