To dread something is to fear something greatly, or to be very reluctant to do something.
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The quotation is exactly as in the original.
"exactly straight"
If angles are congruent they are exactly the same.
Identical means exactly the same:ʌɪˈdɛntɪk(ə)l/adjectiveadjective: identical1. similar in every detail; exactly alike.
If you mean "Does the sum of two angles equal exactly 90 degrees?" it is yes.